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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 05, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Denali Therapeutics Inc.
Entity Central Index Key	0001714899
Trading Symbol	DNLI
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	false
Entity Shell Company	false
Entity Public Float			$ 489.4
Entity Common Stock, Shares Outstanding		95,289,047

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 77,123	$ 218,375
Short-term marketable securities	387,174	187,851
Prepaid expenses and other current assets	16,539	3,381
Total current assets	480,836	409,607
Long-term marketable securities	147,881	60,750
Property and equipment, net	25,162	14,923
Other non-current assets	8,105	1,441
Total assets	661,984	486,721
Current liabilities:
Accounts payable	1,891	2,716
Accrued liabilities	8,520	5,364
Accrued compensation	9,952	5,166
Contract liability	11,427	0
Deferred rent	616	855
Other current liabilities	380	63
Total current liabilities	32,786	14,164
Contract liability, less current portion	57,350	0
Deferred rent, less current portion	24,532	6,294
Other non-current liabilities	471	467
Total liabilities	115,139	20,925
Commitments and contingencies
Convertible preferred stock, $0.01 par value; 40,000,000 shares authorized as of December 31, 2018 and December 31, 2017; 0 shares issued and outstanding as of December 31, 2018 and December 31, 2017	0	0
Stockholders' equity:
Common stock, $0.01 par value; 400,000,000 shares authorized as of December 31, 2018 and December 31, 2017; 94,662,435 shares and 87,480,362 shares issued and outstanding as of December 31, 2018 and December 31, 2017, respectively	1,273	1,201
Additional paid-in capital	774,158	656,660
Accumulated other comprehensive loss	(649)	(368)
Accumulated deficit	(227,937)	(191,697)
Total stockholders’ equity	546,845	465,796
Total liabilities and stockholders’ equity	$ 661,984	$ 486,721

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (usd per share)	$ 0	$ 0
Convertible preferred stock, shares authorized (in shares)	40,000,000	40,000,000
Convertible preferred stock, shares issued (in shares)	0	0
Convertible preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (usd per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common stock, shares issued (in shares)	94,662,435	87,480,362
Common stock, shares outstanding (in shares)	94,662,435	87,480,362

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 129,160	$ 0	$ 0
Operating expenses:
Research and development expense	143,183	74,460	75,702
General and administrative	32,349	15,680	11,731
Total operating expenses	175,532	90,140	87,433
Loss from operations	(46,372)	(90,140)	(87,433)
Interest and other income, net	10,132	1,955	781
Net income (loss)	(36,240)	(88,185)	(86,652)
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities, net of tax	(281)	5	(373)
Comprehensive loss	$ (36,521)	$ (88,180)	$ (87,025)
Net loss per share, basic and diluted (usd per share)	$ (0.39)	$ (5.89)	$ (13.49)
Weighted average number of shares outstanding, basic and diluted (in shares)	92,621,991	14,964,144	6,424,720

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series A-1	Series A-2	Series B-1	Series B-2	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2015		12,197,880
Convertible preferred stock, beginning balance at Dec. 31, 2015		$ 48,308
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net of issuance costs (in shares)			33,916,543	4,361,527	8,124,365
Issuance of convertible preferred stock, net of issuance costs			$ 135,643	$ 34,885	$ 129,837
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2016		58,600,315
Convertible preferred stock, ending balance at Dec. 31, 2016		$ 348,673
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2015							4,260,560
Beginning balance at Dec. 31, 2015	$ (15,634)						$ 170	$ 1,056		$ (16,860)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock as contingent consideration in asset acquisition (in shares)							945,880
Issuance of common stock as contingent consideration in asset acquisition	5,280						$ 38	5,242
Issuance of common stock upon exercise of stock options (in shares)							162,665
Issuance of common stock upon exercise of stock options	111						$ 6	105
Vesting of early exercised common stock (in shares)							239,580
Vesting of early exercised common stock	163						$ 10	153
Vesting of restricted stock awards (in shares)							2,988,631
Vesting of restricted stock awards							$ 120	(120)
Stock-based compensation	2,951							2,951
Net income (loss)	(86,652)									(86,652)
Other comprehensive income (loss)	(373)								$ (373)
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2016							8,597,316
Ending balance at Dec. 31, 2016	(94,154)						$ 344	9,387	(373)	(103,512)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net of issuance costs (in shares)						1,764,705
Issuance of convertible preferred stock, net of issuance costs						$ 29,927
Conversion of Series A and B convertible preferred stock into common stock (in shares)		(60,365,020)
Conversion of Series A and B convertible preferred stock into common stock		$ (378,600)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock as contingent consideration in asset acquisition (in shares)							81,164
Issuance of common stock as contingent consideration in asset acquisition	$ 0						$ 1	(1)
Issuance of common stock upon exercise of stock options (in shares)	695,192						648,317
Issuance of common stock upon exercise of stock options	$ 733						$ 25	708
Vesting of early exercised common stock (in shares)							187,500
Vesting of early exercised common stock	127						$ 6	121
Vesting of restricted stock awards (in shares)							1,628,824
Vesting of restricted stock awards	0						$ 61	(61)
Stock-based compensation	4,409							4,409
Net income (loss)	(88,185)									(88,185)
Other comprehensive income (loss)	5								5
Convertible preferred stock converted into shares of common stock (in shares)							60,365,020
Convertible preferred stock converted into shares of common stock	378,600						$ 604	377,996
Issuance of common stock upon initial public offering, net of issuance costs (in shares)							15,972,221
Shares issued	$ 264,261						$ 160	264,101
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2017	87,480,362						87,480,362
Ending balance at Dec. 31, 2017	$ 465,796						$ 1,201	656,660	(368)	(191,697)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	650,471
Vesting of early exercised common stock (in shares)							234,372
Vesting of early exercised common stock	$ 374						$ 3	371
Vesting of restricted stock awards (in shares)							1,940,203
Vesting of restricted stock awards	0						$ 19	(19)
Stock-based compensation	18,791							18,791
Net income (loss)	(36,240)									(36,240)
Other comprehensive income (loss)	(281)								(281)
Issuance of common stock in connection with Takeda Collaboration Agreement (in shares)							4,214,559
Issuance of common stock in connection with the Takeda Collaboration Agreement	94,406						$ 42	94,364
Issuances under equity incentive plans (in shares)							792,939
Issuances under equity incentive plans	$ 3,999						$ 8	3,991
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2018	94,662,435						94,662,435
Ending balance at Dec. 31, 2018	$ 546,845						$ 1,273	$ 774,158	$ (649)	$ (227,937)

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Series A-1
Convertible preferred stock issuance cost		$ 23
Series A-2
Convertible preferred stock issuance cost		7
Series B-1
Convertible preferred stock issuance cost		$ 153
Series B-2
Convertible preferred stock issuance cost	$ 73
Common Stock
Common stock issuance costs	$ 23,612

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income (loss)	$ (36,240)	$ (88,185)	$ (86,652)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	7,415	3,082	1,469
Stock-based compensation expense	18,791	4,409	2,951
Net amortization of premiums and (discounts) on marketable securities	(2,705)	753	304
Gain (loss) on disposition of property and equipment	(36)	1	3
Fair value of common stock issued in connection with asset acquisition	0	0	5,280
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(18,290)	73	(533)
Accounts payable	(526)	207	161
Accrued and other current liabilities	9,113	3,578	5,357
Deferred rent	3,438	0	0
Contract liability	68,777	0	0
Other non-current liabilities	379	(553)	(248)
Net cash provided by (used in) operating activities	50,116	(76,635)	(71,908)
Investing activities
Purchase of marketable securities	(557,930)	(179,789)	(226,370)
Purchase of property and equipment	(3,393)	(2,875)	(6,134)
Purchase of other investments	0	0	(500)
Maturities and sales of marketable securities	273,901	141,498	14,000
Net cash used in investing activities	(287,422)	(41,166)	(219,004)
Financing activities
Issuance of common stock in connection with the Takeda Collaboration Agreement	94,406	0	0
Proceeds from exercise of common stock options	3,999	733	111
Proceeds from issuance of common stock, net of issuance costs	0	265,619	0
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	29,971	300,365
Net cash provided by financing activities	97,019	296,323	300,476
Net (decrease) increase in cash and cash equivalents	(140,287)	178,522	9,564
Cash, cash equivalents and restricted cash at beginning of year	218,910	40,388	30,824
Cash, cash equivalents and restricted cash at end of year	78,623	218,910	40,388
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	14,561	0	0
Property and equipment purchases accrued but not yet paid	335	103	233
Deferred IPO costs incurred but not yet paid	0	1,358	0
Convertible preferred stock issuance costs incurred but not yet paid	0	44	0
Convertible Preferred Stock
Financing activities
Payments of issuance costs related to issuance of stock	(44)	0	0
Common Stock
Financing activities
Payments of issuance costs related to issuance of stock	$ (1,342)	$ 0	$ 0

Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Significant Accounting Policies

Organization and Description of Business

Denali Therapeutics Inc. ("Denali, or the “Company”) is a biopharmaceutical company, incorporated in Delaware, that discovers and develops therapeutics to defeat neurodegenerative diseases. The Company is headquartered in South San Francisco, California.

Basis of Presentation

These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”).

Reverse Stock Split

In November 2017, the Company’s board of directors and stockholders approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of its common stock and convertible preferred stock on a 4-to-1 basis (the “Reverse Stock Split”). The par values and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split. The Reverse Stock Split became effective on November 28, 2017. All issued and outstanding common stock and convertible preferred stock and related per share amounts contained in the financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented.

Initial Public Offering

On December 7, 2017, the Company's Registration Statement on Form S-1 was declared effective by the SEC for our initial public offering ("IPO") of common stock. The Company's shares started trading on the NASDAQ Global Select Market on December 8, 2017, and the transaction formally closed on December 12, 2017. In connection with the IPO, the Company sold an aggregate of 15,972,221 shares of common stock, including 2,083,333 shares sold pursuant to the underwriters’ full exercise of their option to purchase additional shares, at a price to the public of $18.00 per share. The aggregate net proceeds received by the Company from the offering, net of underwriting discounts and commissions and offering expenses, were $264.3 million. Upon the closing of the IPO, all then-outstanding shares of Company convertible preferred stock converted into 60,365,020 shares of common stock. The related carrying value of $378.6 million was reclassified to common stock and additional paid-in capital. Additionally, the Company amended and restated its certificate of incorporation effective December 7, 2017 to, among other things, change the authorized number of shares of common stock to 400,000,000 shares and the authorized number of shares of preferred stock to 40,000,000 shares.

Principles of Consolidation

These consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated on consolidation. For the Company's subsidiary, the functional currency has been determined to be the U.S. dollar. Monetary assets and liabilities denominated in foreign currency are remeasured at period-end exchange rates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured at historical rates. Foreign currency transaction gains and losses resulting from remeasurement are recognized in Interest and other income, net in the consolidated statements of operations and comprehensive loss, and have not been material.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires the Company to make certain estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements, as well as the reported amounts of expenses during the reporting period. Actual results could differ from those estimates, and such differences could be material to the consolidated financial position and statements of operations and comprehensive loss.

Concentration of Credit Risk and Other Risks and Uncertainties

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. Substantially all of the Company’s cash and cash equivalents are deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits.

The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents and marketable securities and issuers of marketable securities to the extent recorded on the consolidated balance sheets. As of December 31, 2018 and 2017, the Company had no off-balance sheet concentrations of credit risk.

The Company is exposed to counterparty credit risk on all of its derivative financial instruments. The Company has established and maintains strict counterparty credit guidelines and enters into hedges only with financial institutions that are investment grade or better to minimize the Company’s exposure to potential defaults. The Company does not require collateral to be pledged under these agreements.

The Company is subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability.

Segments

The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources.

Fair Value of Financial Instruments

Accounting Standards Codification ("ASC") Topic 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following:

Level 1 – inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities.

Level 2 – inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

Level 3 – inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts reflected in the consolidated balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities approximate their fair values, due to their short-term nature.

Cash and Cash Equivalents

The Company considers all highly liquid investments with original maturities of 90 days or less at the date of purchase to be cash and cash equivalents. Cash equivalents are stated at fair value.

Marketable Securities

The Company generally invests its excess cash in money market funds and investment grade short to intermediate-term fixed income securities. Such investments are included in cash and cash equivalents, short-term marketable securities, or long-term marketable securities on the balance sheets, are considered available-for-sale, and reported at fair value with unrealized gains and losses included as a component of stockholders’ equity (deficit). The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in Interest and other income, net in the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in Interest and other income, net. The cost of securities sold is determined using specific identification.

The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and our strategy and intentions for holding the marketable security.

Restricted Cash

The Company’s restricted cash consists of the letter of credit for the Company’s headquarters building lease, and is included within other non-current assets on the accompanying consolidated balance sheet.

Accounts receivable

Accounts receivable are included within Prepaid expenses and other current assets in the consolidated balance sheets. The accounts receivable balance represents amounts receivable from collaboration partners. The Company estimates the allowance for doubtful accounts based on existing contractual payment terms, actual payment patterns of its partners, and individual partner circumstances. To date, an allowance for doubtful accounts has not been required.

Derivatives and Hedging Activities

The Company accounts for its derivative instruments as either assets or liabilities on the consolidated balance sheet and measures them at fair value. Derivatives are adjusted to fair value through Interest and other income, net in the consolidated statements of operations and comprehensive loss.

Property and Equipment, Net

Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is computed using the straight-line method over the related estimated useful lives as presented in the table below. Significant additions and improvements are capitalized, while repairs and maintenance are charged to expense as incurred.


Leasehold improvements			Shorter of life of asset or lease term
Manufacturing and laboratory equipment			five years
Computer hardware and software			three years
Office furniture and equipment			five years

Impairment of Long-Lived Assets

The Company periodically evaluates property and equipment for impairment whenever events or changes in circumstances indicate that a potential impairment may have occurred. If such events or changes in circumstances arise, the Company compares the carrying amount of the long-lived assets to the estimated future undiscounted cash flows expected to be generated by the long-lived assets. If the estimated aggregate undiscounted cash flows are less than the carrying amount of the long-lived assets, an impairment charge, calculated as the amount by which the carrying amount of the assets exceeds the fair value of the assets, is recorded. The fair value of the long-lived assets is determined based on the estimated discounted cash flows expected to be generated from the long-lived assets. The Company has not recorded any such impairment charges during the years presented.

Deferred Rent

Certain of the Company’s operating lease agreements include scheduled rent escalations over the lease term, as well as tenant improvement allowances. Rent expense is charged ratably over the life of the lease. Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies. Tenant improvement allowances are recorded as a deferred rent liability and are amortized on a straight-line basis over the term of the lease as a reduction to rent expense.

Revenue Recognition

License and Collaboration Revenue

The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements (“ASC 808”) to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of Topic 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. The accounting treatment pursuant to Topic 606 is outlined below.

The terms of licensing and collaboration agreements entered into typically include payment of one or more of the following: nonrefundable, up-front license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and royalties on net sales of licensed products. Each of these payments results in license, collaboration and other revenue, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenue. The core principle of Topic 606 is to recognize revenue when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Amounts received prior to satisfying the revenue recognition criteria are recorded as contract liabilities in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company's consolidated balance sheets. If the Company expects to have an unconditional right to receive the consideration in the next twelve months, this will be classified in current assets. A net contract asset or liability is presented for each contract with a customer.

At contract inception, the Company assesses the goods or services promised in a contract with a customer and identifies those distinct goods and services that represent a performance obligation. A promised good or service may not be identified as a performance obligation if it is immaterial in the context of the contract with the customer, if it is not separately identifiable from other promises in the contract (either because it is not capable of being separated or because it is not separable in the context of the contract), or if the performance obligation does not provide the customer with a material right.

The Company considers the terms of the contract to determine the transaction price. The transaction price is the amount of consideration to which the Company expects to be entitled in exchange for transferring promised goods or services to a customer. The consideration promised in a contract with a customer may include fixed amounts, variable amounts, or both. Variable consideration will only be included in the transaction price when it is not considered constrained, which is when it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur.

If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling prices ("SSP"). The relative SSP for each deliverable is estimated using external sourced evidence if it is available. If external sourced evidence is not available, the Company uses its best estimate of the SSP for the deliverable.

Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset, which for a service is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the service promised to the customer.

After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the transaction price is allocated to the performance obligations on the same basis as at contract inception.

Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenue, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations.

Research and Development Expenses

Research and development costs are expensed as incurred. Research and development costs consist of salaries and other personnel related expenses, including associated stock–based compensation, consulting fees, lab supplies, and facility costs, as well as fees paid to other entities that conduct certain research, development and manufacturing activities on behalf of the Company.

Nonrefundable advance payments for goods and services that will be used or received in future research and development activities are deferred and recognized as expense in the period in which the related goods are delivered or services are performed.

The Company has acquired and may continue to acquire the rights to develop and commercialize new product candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that there is no alternative future use of the rights in other research and development projects.

Stock-Based Compensation

The Company’s stock-based compensation programs grant awards that have included stock options, restricted stock units, restricted stock awards, and shares issued under its employee stock purchase plan. Grants are awarded to employees, including directors, and non-employees.

The Company measures employee and director stock-based compensation expense for all stock-based awards at the grant date based on the fair value measurement of the award. Subsequent to the adoption of ASU No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting on September 30, 2018, future stock-based compensation expense for non-employee stock-based awards is measured based on the fair value on the date of adoption. The expense is recorded on a straight-line basis over the requisite service period, which is generally the vesting period, for the entire award. Expense is adjusted for actual forfeitures of unvested awards as they occur. The Company calculates the fair value measurement of stock options using the Black-Scholes valuation model. The Company uses the fair value of our common stock to determine the fair value of restricted stock awards.

The Company granted restricted stock awards that vest in conjunction with certain performance conditions to certain key employees and directors. At each reporting date, the Company is required to evaluate whether achievement of the performance conditions is probable. Compensation expense is recorded over the appropriate service period based upon the Company’s assessment of accomplishing each performance provision.

Income Taxes

Income taxes are accounted for using the liability method, under which deferred tax assets and liabilities are determined based on the temporary differences between the financial reporting and tax bases of assets and liabilities and consideration is given to net operating losses and tax credit carryforwards. Deferred tax assets and liabilities are measured using the enacted tax rates that are expected to be in effect when the differences are expected to reverse.

The Company assesses the likelihood that deferred tax assets will be recovered from future taxable income, and a valuation allowance is established when necessary to reduce deferred tax assets to the amounts more likely than not expected to be realized.

The Company recognizes and measures uncertain tax positions using a two–step approach. The first step is to evaluate the tax position taken or expected to be taken by determining whether the weight of available evidence indicates that it is more likely than not that the tax position will be sustained in an audit, including resolution of any related appeals or litigation processes. The second step is to measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. Significant judgment is required to evaluate uncertain tax positions. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. Previously recognized tax positions that no longer meet the more-likely-than-not threshold are derecognized in the first subsequent financial reporting period in which that threshold is no longer met.

The Company’s policy is to recognize interest and penalties accrued on any unrecognized tax benefits as a component of income tax expense.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss, primarily unrealized gains or losses on the Company’s marketable securities.

Net Income (Loss) per Share

Basic net income (loss) per share is calculated by dividing the net income (loss) by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents.

Diluted net income per ordinary share is computed by giving effect to all dilutive potential ordinary shares including options. However, where there is a diluted net loss per ordinary share, no adjustment is made for potentially issuable ordinary shares since their effect would be anti-dilutive. In this case, diluted net loss per share is equal to basic net loss per share.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update ("ASU") No. 2016-02, Leases (Topic 842), which supersedes the guidance in former ASC 840, Leases. The FASB issued further updates to this guidance in July 2018 through ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements, in December 2018 through ASU 2018-20, Leases (Topic 842): Narrow-Scope Improvements for Lessors and in March 2019 through ASU 2019-01 Leases (Topic 842): Codification Improvements. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. The standard is effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and is required to be adopted using a modified retrospective approach.

The Company is substantially complete with its evaluation of the effect that the adoption of this ASU will have on its consolidated financial statements. The Company plans to adopt this standard on January 1, 2019 applying the optional transition method such that it is not required to adjust prior period presentations. ASU 2016-02 is expected to impact the Company’s consolidated financial statements as the Company has certain operating lease arrangements for which the Company is the lessee and one future operating lease arrangement for which the Company is the lessor. The Company has no financing leases. Management plans to elect the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows us to carryforward the historical lease classification. The estimated impact of adoption of the standard is that the Company will recognize a net right of use asset and incremental lease liabilities of approximately $46.1 million, as of January 1, 2019. Management still does not expect the adoption to have a material change to the consolidated statements of operations and comprehensive loss, stockholders' equity (deficit), or cash flows.

Recently Adopted Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014–09, Revenue from Contracts with Customers (Topic 606), which amends the existing accounting standards for revenue recognition. The FASB issued further updates to this guidance through ASU 2016-12 Narrow-Scope Improvements and Practical Expedients, ASU 2016-10 Identifying Performance Obligations and Licensing and ASU 2016-08 Principal Versus Agent Considerations (Reporting Revenue Gross Versus Net). The new standard is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This standard was adopted on January 1, 2018 using a full retrospective application. There was no impact to the consolidated financial statements upon adoption of ASU 2014-09 as the Company had not recognized any revenue through December 31, 2017.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The purpose of ASU 2016-18 is to clarify the guidance for and presentation of restricted cash in the statement of cash flows. The amendment requires beginning-of-period and end-of-period total amounts shown on the statement of cash flows to include cash and cash equivalents as well as restricted cash and restricted cash equivalents. This standard was adopted on January 1, 2018. Accordingly, the consolidated statements of cash flows and Note 3 "Cash and Marketable Securities" have been updated to reconcile cash, cash equivalents and restricted cash for all periods presented.

In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting, which clarifies when to account for a change to the terms or conditions of a stock-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. This standard was adopted as of January 1, 2018 and will be applied prospectively to any award modified after the adoption date.

In June 2018, the FASB issued ASU No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 is intended to reduce the cost and complexity and to improve financial reporting for non-employee stock-based payments. ASU 2018-07 expands the scope of Topic 718, Compensation-Stock Compensation (which currently only includes stock-based payments to employees) to include stock-based payments issued to non-employees for goods or services. Consequently, the accounting for stock-based payments to non-employees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity-Based Payments to Non-Employees. The Company elected to early adopt this standard effective September 30, 2018. The new guidance is applied to all new equity-classified stock-based payment awards issued to non-employees after the date of adoption. In addition, for all previously issued equity-classified stock-based payment awards to non-employees for which a measurement date was not established by the adoption date, these awards were remeasured at fair value as of the adoption date and will no longer be remeasured. The future expense for these stock-based payment awards to non-employees will be based on the fair value as of the adoption date. The adoption of this standard did not result in any change to the consolidated financial statements.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The standard adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with customer revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company has early adopted this standard as of December 31, 2018 and applied the standard retrospectively to January 1, 2018, the date of initial application of Topic 606. The new standard has been applied to all contracts, and there was no impact to the consolidated financial statements upon adoption.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

Assets and liabilities measured at fair value at each balance sheet date are as follows (in thousands):


	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	42,225	$	—	$	—	$	42,225
U.S. government treasuries	1,499		—		—		1,499
Commercial paper	—		9,979		—		9,979
Short-term marketable securities:
U.S. government treasuries	219,754		—		—		219,754
U.S. government agency securities	—		73,151		—		73,151
Corporate debt securities	—		71,675		—		71,675
Commercial paper	—		22,594		—		22,594
Long-term marketable securities:
U.S. government treasuries	117,131		—		—		117,131
U.S. government agency securities	—		1,977		—		1,977
Corporate debt securities	—		28,773		—		28,773
Foreign currency derivative contracts	—		14		—		14
Total	$	380,609	$	208,163	$	—	$	588,772
Liabilities:
Foreign currency derivative contracts	$	—	$	182	$	—	$	182
Total	$	—	$	182	$	—	$	182


	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents
Money market funds	$	212,868	$	—	$	—	$	212,868
Short-term marketable securities:
U.S. government treasuries	42,587		—		—		42,587
U.S. government agency securities	—		106,139		—		106,139
Corporate debt securities	—		39,125		—		39,125
Long-term marketable securities:
U.S. government treasuries	39,848		—		—		39,848
U.S. government agency securities	—		19,911		—		19,911
Corporate debt securities	—		991		—		991
Total	$	295,303	$	166,166	$	—	$	461,469

The carrying amounts of accounts receivable, accounts payable and accrued liabilities approximate their fair values due to their short-term maturities.

The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly.

There were no transfers of assets or liabilities between the fair value measurement levels during the years ended December 31, 2018 or 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Cash and Marketable Securities

12 Months Ended

Dec. 31, 2018

Cash and Cash Equivalents [Abstract]

Cash and Marketable Securties

Cash and Marketable Securities

Cash, cash equivalents and restricted cash

A reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets to the amount reported within the consolidated statements of cash flows is shown in the table below (in thousands):


	December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	77,123	$	218,375	$	39,853
Restricted cash included within prepaid expenses and other current assets	—		84		—
Restricted cash included within other non-current assets	1,500		451		535
Total cash, cash equivalents, and restricted cash	$	78,623	$	218,910	$	40,388

Marketable Securities

All marketable securities were considered available-for-sale at December 31, 2018 and 2017. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands):


	December 31, 2018
	Amortized Cost		Unrealized Holding Gains		Unrealized Holding Losses		Aggregate Fair Value
Short-term marketable securities:
U.S. government treasuries	$	220,081	$	29	$	(356)	$	219,754
U.S. government agency securities	73,373		—		(222)		73,151
Corporate debt securities	71,940		1		(266)		71,675

Commercial paper	22,594		—		—		22,594
Total short-term marketable securities	387,988		30		(844)		387,174
Long-term marketable securities:
U.S. government treasuries	116,878		329		(76)		117,131
U.S. government agency securities	1,975		2		—		1,977
Corporate debt securities	28,864		8		(99)		28,773
Total long-term marketable securities	147,717		339		(175)		147,881
Total	$	535,705	$	369	$	(1,019)	$	535,055


	December 31, 2017
	Amortized Cost		Unrealized Holding Gains		Unrealized Holding Losses		Aggregate Fair Value
Short-term marketable securities:
U.S. government treasuries	$	42,614	$	—	$	(27)	$	42,587
U.S. government agency securities	106,368		—		(229)		106,139
Corporate debt securities	39,197		—		(72)		39,125
Total short-term marketable securities	188,179		—		(328)		187,851
Long-term marketable securities:
U.S. government treasuries	39,868		—		(20)		39,848
U.S. government agency securities	19,931		—		(20)		19,911
Corporate debt securities	991		—		—		991
Total long-term marketable securities	60,790		—		(40)		60,750
Total	$	248,969	$	—	$	(368)	$	248,601

As of December 31, 2018 and 2017, some of the Company’s marketable securities were in an unrealized loss position. At each balance sheet date, the Company determined that it did have the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery, thus there has been no recognition of any other-than-temporary impairment in the years ended December 31, 2018, 2017 or 2016. All marketable securities with unrealized losses as of each balance sheet date have been in a loss position for less than twelve months or the loss is not material.

All of the Company’s marketable securities have an effective maturity of less than two years.

Derivative Financial Instruments

12 Months Ended

Dec. 31, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Financial Instruments

Foreign Currency Exchange Rate Exposure

The Company uses forward foreign currency exchange contracts to hedge exposure to potential changes in foreign currency exchange rates. Such exposures result from certain external research and development activities taking place in foreign countries for which forecasted cash flows are denominated in currencies other than the U.S. dollar, primarily the Euro and British Pound. The derivative instruments the Company uses to hedge this exposure are not designated as cash flow hedges, and as a result, changes in their fair value are recorded in Interest and other income, net, on the Company's consolidated statements of operations and comprehensive loss.

The fair values of forward foreign currency exchange contracts are estimated using current exchange rates and interest rates and take into consideration the current creditworthiness of the counterparties. Information regarding the specific instruments used by the Company to hedge its exposure to foreign currency exchange rate fluctuations is provided below. The Company did not have foreign currency exchange contracts prior to June 2018.

The following table summarizes the Company’s forward foreign currency exchange contracts outstanding as of December 31, 2018 (notional amounts in thousands):

Foreign Exchange Contracts

Number of Contracts

Aggregate Notional⁽¹⁾ Amount in Foreign Currency

Maturity

Euros

2,343

Jan. 2019 - Nov. 2019

British Pounds

4,017

Jan. 2019 - Nov. 2019

Swiss Francs

1,100

Jan. 2019 - Nov. 2019

Total

_________________________________________________

⁽¹⁾The notional amount represents the net amount of foreign currency that will be received upon maturity of the forward contracts.

The derivative liability balance of $0.2 million is recorded in Other current liabilities and the derivative asset balance of $13,669 is recorded in Prepaid assets and other current assets on the consolidated balance sheet as of December 31, 2018. The Company recognized a net loss on forward foreign currency exchange contracts of $0.2 million in Interest and other income, net in the consolidated statement of operations and comprehensive loss for the year ended December 31, 2018.

Acquisition	12 Months Ended
Acquisition	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Acquisition	Acquisition In August 2016, the Company entered into a License and Collaboration Agreement (“F-star Collaboration Agreement”) with F-star Gamma Limited (“F-star Gamma”), F-star Biotechnologische Forschungs-Und Entwicklungsges M.B.H ("F-star GmbH") and F-star Biotechnology Limited ("F-star Ltd") (collectively, “F-star”) to leverage F-star’s modular antibody technology and the Company’s expertise in the development of therapies for neurodegenerative diseases. In connection with the entry into the F-star Collaboration Agreement, the Company also purchased an option for an upfront option fee of $0.5 million (the “buy-out-option”), to acquire all of the outstanding shares of F-star Gamma pursuant to a pre-negotiated buy-out option agreement (the “Option Agreement”). In May 2018, the Company exercised the Option Agreement and entered into a Share Purchase Agreement (the “Purchase Agreement”) with the shareholders of F-star Gamma and Shareholder Representative Services LLC, pursuant to which the Company acquired all of the outstanding shares of F-star Gamma (the “Acquisition”). As a result of the Acquisition, F-star Gamma has become a wholly-owned subsidiary of the Company and the Company has changed the entity’s name to Denali BBB Holding Limited. In addition, the Company became a direct licensee of certain intellectual property of F-star Ltd (by way of the Company’s assumption of F-star Gamma’s license agreement with F-star Ltd, dated August 24, 2016, (the “F-star Gamma License”)). The Company has made initial exercise payments under the Purchase Agreement and the F-star Gamma License in the aggregate, of $18.0 million, less the estimated net liabilities of F-star Gamma, which is approximately $0.2 million. In addition, the Company is required to make future contingent payments, to F-star Ltd and the former shareholders of F-star Gamma, up to a maximum amount of $447.0 million in the aggregate upon the achievement of certain defined preclinical, clinical, regulatory and commercial milestones. The amount of the contingent payments varies based on whether F-star delivers an Fcab (constant Fc-domains with antigen-binding activity) that meets pre-defined criteria and whether the Fcab has been identified solely by the Company or solely by F-star or jointly by the Company and F-star. Under the terms of the original F-star Collaboration Agreement, the Company could nominate up to three Fcab targets (“Accepted Fcab Targets”) within the first three years of the date of the F-star Collaboration Agreement. Upon entering into the F-star Collaboration Agreement, the Company had selected transferrin receptor (“TfR”) as the first Accepted Fcab Target and paid F-star Gamma an upfront fee of $5.5 million, which included selection of the first Accepted Fcab Target. In May 2018, the Company exercised its right to nominate two additional Fcab Targets and identified a second Accepted Fcab Target. The Company made a one-time payment for the two additional Accepted Fcab Targets of, in the aggregate, $6.0 million and has extended the time period for its selection of the third Accepted Fcab Target until approximately the fourth anniversary of the date of the original F-star Collaboration Agreement. The Company concluded that the assets acquired and liabilities assumed upon the exercise of the Option Agreement did not meet the accounting definition of a business, and as such, the acquisition was accounted for as an asset purchase. The Company recorded the upfront purchase price less estimated net liabilities acquired of $17.8 million in research and development expense in the accompanying consolidated statement of operations and comprehensive loss in the year ended December 31, 2018 since it represented consideration for in-process research and development with no future alternative use. The upfront option fee of $0.5 million previously included within other non-current assets was also included in research and development expense during the year ended December 31, 2018. This transaction was accounted for as an asset purchase rather than a business combination, and the Company did not recognize any contingent consideration on the acquisition date. Contingent consideration is expected to be recognized in research and development expense in the future as incurred. The Company was and continues to be responsible for certain research costs incurred by F-star Ltd in conducting activities under each agreed development plan, for up to 24 months. The research costs for the agreed TfR development plan was $2.1 million. The Company recognized $0.7 million, $1.1 million, and $0.3 million of research and development expense related to the funding of F-star Ltd activities under the TfR development plan during the years ended December 31, 2018, 2017, and 2016, respectively.

Collaboration Agreements

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaboration Agreements

Sanofi

In October 2018, the Company entered into a Collaboration and License Agreement ("Sanofi Collaboration Agreement") with Genzyme Corporation, a wholly owned subsidiary of Sanofi S.A. ("Sanofi") pursuant to which certain small molecule CNS and peripheral RIPK1 inhibitors contributed by Sanofi and by Denali will be developed and commercialized. The Sanofi Collaboration Agreement became effective in November 2018 when the HSR requirements were satisfied upon which Sanofi paid the Company an upfront payment of $125.0 million. Under the Sanofi Collaboration Agreement, Denali is eligible to receive milestone payments from Sanofi up to approximately $1.1 billion upon achievement of certain clinical, regulatory and sales milestone events. Such milestone payments include $600.0 million in clinical and regulatory milestone payments for CNS Products and $495.0 million in clinical, regulatory and commercial milestone payments for Peripheral Products, as defined.

Denali will share profits and losses equally with Sanofi for CNS Products sold in the United States and China, and receive royalties on net sales for CNS Products sold outside of the United States and China and for Peripheral Products sold worldwide, each as further described below. RIPK1 Inhibitors contributed by Sanofi and developed and commercialized under the Sanofi Collaboration Agreement will be subject to lower milestone and royalty payments to Denali compared to RIPK1 Inhibitors contributed by Denali.

Denali and Sanofi will jointly develop CNS Products pursuant to a global development plan. The Company will be responsible, at its own cost, for conducting Phase 1 and Phase 2 trials for CNS Products for Alzheimer’s disease and any activities required to support such clinical trials and specific for Alzheimer’s disease. Denali will also conduct, at Sanofi’s cost, a Phase 1b trial for the lead CNS penetrant RIPK1 inhibitor, DNL747, for ALS. Sanofi will be responsible, at its cost, for all other Phase 1 and Phase 2 trials for CNS Products, including for multiple sclerosis. Sanofi will lead the conduct of all Phase 3 and later stage development trials for CNS Products, with Sanofi funding 70% of such costs and Denali funding 30% of such costs. The Company will have the ability to opt out of the cost-profit sharing provisions of the Sanofi Collaboration Agreement, as further described below.

Sanofi will lead commercialization activities globally for CNS Products. The Company may elect to conduct certain co-commercialization activities outside of MS with respect to each CNS Product in the United States and/or China, provided that the cost-profit sharing provisions of the Sanofi Collaboration Agreement for the relevant CNS Product are still in effect, as further described below.

The Company may opt out of the cost-profit sharing provisions of the Sanofi Collaboration Agreement for CNS Products in the United States and China on a CNS Product and country basis. Sanofi may also terminate Denali's cost-profit sharing provisions of the Sanofi Collaboration Agreement in its entirety if, following notice from Sanofi and a cure period, the Company fails to satisfy its cost-sharing obligations. After such an opt out by the Company or termination by Sanofi, Denali will no longer be obligated to share in the development and commercialization costs for the applicable CNS Products and Denali will not share in the applicable profits from such CNS Products. Instead, the Company will be entitled to receive tiered royalties on net sales of the applicable CNS Products in the relevant country (or countries). The royalty rates are a percentage in the low double digits to mid-teens, but may increase to the mid-teens to low-twenties percentages for all countries in which Sanofi is paying royalties on the applicable CNS Products, if the Company has met certain co-funding thresholds at the time of its election or Sanofi’s termination of the Company's cost-profit sharing rights and obligations.

Sanofi will be responsible, at its cost, for conducting activities relating to the development and commercialization of all Peripheral Products. Sanofi will lead commercialization activities globally for Peripheral Products. Denali will be entitled to receive tiered royalties in the low- to mid- teen percentages on net sales of Peripheral Products.

The Company identified the following distinct performance obligations associated with the Sanofi Collaboration Agreement upon inception: the CNS program license, the Peripheral program license, the Phase 1 and Phase 2 trials for CNS Products for Alzheimer’s disease ("Alzheimer's Disease Services"), and the Phase 1b trial for DNL747 for ALS and associated activities ("Retained Activities").

The Company believes that the Sanofi Collaboration Agreement is a collaboration arrangement as defined in ASC 808, Collaborative Agreements. The Company also believes that Sanofi meets the definition of a customer as defined in ASC 606, Revenue From Contracts With Customers for three of the performance obligations identified at inception, but does not meet the definition of a customer for the Alzheimer's Disease Services. Further, Sanofi does not meet the definition of a customer for all Phase 3 and later stage development trials for CNS Products led by Sanofi for which Denali will fund 30% of total costs. Since ASC 808 does not address recognition and measurement, the Company looked to other accounting literature for guidance where the performance obligation does not fall under ASC 606, and determined that for the Alzheimer's Disease Services, the guidance in ASC 606 should be analogized for the recognition, measurement and reporting of this performance obligation, and for the cost sharing provisions, the Company determined that the guidance in ASC 730, Research and Development should be applied.

The transaction price at inception included upfront fixed consideration of $125.0 million. All potential future milestones and other payments were considered constrained at the inception of the Sanofi Collaboration Agreement since the Company could not conclude it is probable that a significant reversal in the amount recognized will not occur. The transaction price increased by $2.3 million from inception through December 31, 2018 as payments for costs incurred related to the Retained Activities and associated activities were no longer constrained.

The respective standalone value for each of the performance obligations has been determined by applying the SSP method and the transaction price allocated based on the relative SSP method with revenue recognition timing to be determined either by delivery or the provision of services.

The Company used an adjusted market assessment approach to estimate the selling price for the program licenses, and an expected cost plus margin approach for estimating the Alzheimer’s Disease Services, and the Retained Activities. The program licenses and existing know-how were delivered on the effective date of the Sanofi Collaboration Agreement. The Alzheimer’s Disease Services and the Retained Activities are expected to be delivered over time as the services are performed. For the Alzheimer's Disease Services, revenue will be recognized over time using the input method, based on costs incurred to perform the services, since the level of costs incurred over time is thought to best reflect the transfer of services to Sanofi. For the Retained Activities, revenue will be recognized over time using the output method, based on amounts invoiced to Sanofi, since this is believed to directly correlate to the value of the services performed.

A contract liability of $3.9 million is recorded on the balance sheet at December 31, 2018, which relates to the portion of the Alzheimer's Disease Services performance obligation yet to be satisfied, with such amounts to be recognized over the estimated period of the services, which is expected to be several years.

There is a receivable of $2.3 million as of December 31, 2018 associated with the Sanofi Collaboration Agreement.

In assessing the Sanofi Collaboration Agreement, management was required to exercise considerable judgment in estimating revenue to be recognized. Management applied judgment in determining the separate performance obligations, in estimating the selling price, in determining when control was transferred to Sanofi for the licenses, and in estimating total future costs when using the input method.

Through December 31, 2018, Denali has not achieved any milestones or recorded any product sales under the Sanofi Collaboration Agreement.

Takeda

In January 2018, the Company entered into a Collaboration and Option Agreement ("Takeda Collaboration Agreement") with Takeda Pharmaceutical Company Limited ("Takeda"), pursuant to which the Company granted Takeda an option in respect of programs to develop and commercialize, jointly with the Company, certain biologic products that are enabled by three Denali's blood brain barrier ("BBB") delivery technology and intended for the treatment of neurodegenerative disorders. The programs were Denali’s ATV:BACE1/Tau and ATV: TREM2 programs, as well as a third identified discovery stage program. The Takeda Collaboration Agreement became effective in February 2018 when the requirements of the Hart-Scott-Rodino Antitrust Improvements Act of 1976 were satisfied. In February 2019, the agreement was amended to replace ATV:BACE1/Tau with ATV:Tau. The amendment did not have a material impact to the consolidated financial statements.

Under the Takeda Collaboration Agreement and unless otherwise agreed jointly between both parties, Denali will be responsible, at its cost, for conducting activities relating to pre-IND development of biologic products directed to the three identified targets and enabled by its BBB delivery technology targeting TfR during the applicable research period. The period through which the option can be exercised continues for each target until the first biologic product directed to the relevant target is IND-ready or approximately five years after selection of the target, whichever is earlier.

Under the Takeda Collaboration Agreement, Takeda paid a $40.0 million upfront, and may pay up to an aggregate of $25.0 million with respect to each program directed to a target and based upon the achievement of certain preclinical milestone events, up to $75.0 million in total. The upfront payment of $40.0 million was received in February 2018, as well as the first preclinical milestone payment of $5.0 million related to one of the programs.

If Takeda exercises its option with respect to a particular target, then Takeda will have the right to develop and commercialize, jointly with the Company, a specified number of biologic products enabled by its BBB delivery technology that were developed during the research period and which are directed to the relevant target, and the Company will grant to Takeda a co-exclusive license under the intellectual property the Company controls related to those biologic products.

Takeda is obligated to pay Denali a $5.0 million option fee for each target for which Takeda exercises its option, up to $15.0 million in total.

In addition, Takeda may be obligated to pay Denali up to an aggregate of $707.5 million upon achievement of certain clinical and regulatory milestone events if Takeda exercises its option for all three collaboration programs. Takeda may also be obligated to pay Denali up to $75.0 million per biologic product upon achievement of a certain sales-based milestone, or an aggregate of $225.0 million if one biologic product from each program achieves this milestone.

If Takeda exercises its option for a particular target, Denali and Takeda will share equally the development and commercialization costs, and, if applicable, the profits, for each collaboration program. However, for each collaboration program, the Company may elect not to continue sharing development and commercialization costs, or Takeda may elect to terminate Denali's cost-profit sharing rights and obligations if, following notice from Takeda and a cure period, the Company fails to satisfy its cost sharing obligations with respect to the relevant collaboration program. After such an election by the Company or termination by Takeda becomes effective, Denali will no longer be obligated to share in the development and commercialization costs for the relevant collaboration program, and will not share in any profits from that collaboration program. Instead the Company will be entitled to receive tiered royalties. The royalties will be in the low- to mid-teen percentages on net sales, or low- to high-teen percentages on net sales if certain co-funding thresholds have been met at the time of the Company's election to opt out of co-development or Takeda’s termination of Denali's cost-profit sharing rights and obligations, and, in each case, these royalty rates will be subject to certain reductions specified in the Takeda Collaboration Agreement. Takeda will pay these royalties for each biologic product included in the relevant collaboration program, on a country-by-country basis, until the latest of (i) the expiration of certain patents covering the relevant biologic product, (ii) the expiration of all regulatory exclusivity for that biologic product, and (iii) an agreed period of time after the first commercial sale of that biologic product in the applicable country, unless biosimilar competition in excess of a significant level specified in the Takeda Collaboration Agreement occurs earlier, in which case Takeda’s royalty obligations in the applicable country would terminate.

For each collaboration program for which costs and profits are shared with Takeda, Denali will lead the conduct of clinical activities for each indication through the first Phase 2 trial, and Takeda will lead the conduct of all subsequent clinical activities for that indication. Further, Denali and Takeda will jointly commercialize biologic products included in the relevant collaboration program in the United States and China. Unless Denali has opted out of cost-sharing for two collaboration programs, it has the right to lead commercialization activities in the United States for one collaboration program and Takeda will lead commercialization activities in the United States for all collaboration programs for which Denali does not lead commercialization activities. Further, Takeda will lead commercialization activities in China and will solely conduct commercialization activities in all other countries. The Company has the right to lead all manufacturing activities for all collaboration programs for which the parties are sharing costs and profits.

Each party may terminate the Takeda Collaboration Agreement in its entirety, or with respect to a particular collaboration program, as applicable, if the other party remains in material breach of the Takeda Collaboration Agreement following a cure period to remedy the material breach. Takeda may terminate the Takeda Collaboration Agreement in its entirety or with respect to any particular collaboration program, for convenience and after giving a specified amount of prior notice, but Takeda may not do so for a certain period of time after the Effective Date of the Takeda Collaboration Agreement. Takeda may also terminate the Takeda Collaboration Agreement with respect to any collaboration program if the joint steering committee ("JSC") established under the Takeda Collaboration Agreement unanimously agrees that a material safety event has occurred with respect to the applicable collaboration program. Denali may terminate the Takeda Collaboration Agreement with respect to a particular collaboration program if Takeda fails to conduct material development and commercial activities for a specified period of time with respect to a collaboration program, unless Takeda cures such failure within a certain period of time. Denali and Takeda may each terminate the Takeda Collaboration Agreement in its entirety if the other party is declared insolvent or in similar financial distress or if, subject to a specified cure period, the other party challenges any patents licensed to it under the Takeda Collaboration Agreement.

Pursuant to the terms of the Takeda Collaboration Agreement, the Company entered into a common stock purchase agreement (the "Stock Purchase Agreement") with Takeda on January 3, 2018, pursuant to which Takeda purchased 4,214,559 shares of Denali’s common stock (the "Shares") for an aggregate purchase price of $110.0 million. The sale of the Shares closed on February 23, 2018. The fair market value of the common stock sold to Takeda was $94.4 million, based on the closing stock price of $22.40 on the date of issuance, resulting in a $15.6 million premium paid to the Company above the fair value of the Company's common stock which was credited to contract liability in the Company's consolidated balance sheet.

The Company believes that the Takeda Collaboration Agreement is a collaboration arrangement as defined in ASC 808, Collaborative Agreements. Further, during the research period, the Company believes that the arrangement is a contract with a customer as defined in ASC 606, Revenue From Contracts With Customers. The Takeda Collaboration Agreement and the Stock Purchase Agreement are being accounted for as one arrangement because they were entered into at the same time with interrelated financial terms.

The Company identified performance obligations during the research period consisting of the license, the development options, and JSC participation together with the research services for each collaboration program. The license rights, JSC involvement, option and research services are considered to be a single performance obligation for each program since the research services are highly interrelated with the option and JSC involvement and will significantly modify the license. The performance obligations under each of the three programs are separate since the activities and risks under the programs are distinct.

The Company has determined that all other goods or services which are contingent upon Takeda exercising its option for each program are not considered performance obligations at the inception of the Takeda Collaboration Agreement.

The transaction price at inception included fixed consideration consisting of the upfront fee of $40.0 million, the $15.6 million premium on the sale of common stock, and the first preclinical milestone payment of $5.0 million. It also included variable consideration of $26.0 million relating to future milestones that were not constrained. The amount of variable consideration was estimated using the most likely amount method. In October 2018, the Company confirmed the first preclinical milestone was met for the second program, triggering a milestone payment of $5.0 million, which was received in November 2018. In December 2018, the Company confirmed the first preclinical milestone was met for the third program, triggering a milestone payment of $5.0 million, which is recorded within Prepaid expenses and other current assets on the consolidated balance sheet at December 31, 2018. The payment was received in February 2019.

The remaining $44.0 million of preclinical milestones were considered constrained at the inception of the Takeda Collaboration Agreement since the Company could not conclude it is probable that a significant reversal in the amount recognized will not occur. Additionally, cost and profit sharing income, and the development and commercial milestones as outlined above, have not been considered given Takeda has not exercised its options for the development and commercial phases for each program. There was no change in the transaction price from inception through December 31, 2018. This will be reassessed at each reporting period.

The transaction price has been ascribed in its entirety to the three performance obligations identified in the research term of the Takeda Collaboration Agreement.

Revenue is recognized when, or as, the Company satisfies its performance obligations by transferring the promised services to Takeda. Revenue will be recognized over time using the input method, based on costs incurred to perform the research services, since the level of costs incurred over time is thought to best reflect the transfer of services to Takeda. There were no material changes in estimates during the year ended December 31, 2018.

A contract liability of $64.9 million is recorded on the balance sheet at December 31, 2018, which relates to the three performance obligations identified, with such amounts to be recognized over the estimated period of the pre-IND research services, which is expected to be several years.

Revenue recognized relating to future milestone payments of approximately $1.7 million, for which the Company concluded that it is probable that a significant reversal in the amount recognized will not occur, is presented net in the contract liability on the consolidated balance sheet.

Significant changes in the net contract liability balance during the year ended December 31, 2018 are as follows (in thousands):


	Contract liability
Balance at January 1, 2018	$	—
Increases due to cash received, excluding amounts recognized as revenue during the period	66,619
Decreases due to revenue recognized in the period for which cash has not been received	(1,701)
Balance at December 31, 2018	$	64,918

There is a receivable of $5.0 million as of December 31, 2018 associated with the Takeda Collaboration Agreement.

In assessing the Takeda Collaboration Agreement, management was required to exercise considerable judgment in estimating revenue to be recognized. Management applied judgment in determining the separate performance obligations in the research period, estimating variable consideration, and estimating total future costs when using the input method.

Through December 31, 2018, Denali has recognized $15.0 million in milestones from Takeda and has not recorded any product sales under the Takeda Collaboration Agreement.

Collaboration Revenue

Revenue disaggregated by collaboration agreement and performance obligation for the year ended December 31, 2018 is as follows (in thousands):


	Revenue
Takeda Collaboration Agreement	$	5,677
Sanofi Collaboration Agreement:
CNS program license	73,932
Peripheral program license	47,148
Alzheimer's Disease Services	60
Retained Activities	2,343
Total Sanofi Collaboration Revenue	123,483
Total Collaboration Revenue	$	129,160

There was no collaboration revenue in the years ended December 31, 2017 and 2016.

License Agreements	12 Months Ended
License Agreements	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License Agreements	License Agreements Genentech In June 2016, the Company entered into an Exclusive License Agreement with Genentech, Inc. (“Genentech”). The agreement gives the Company access to Genentech’s LRRK2 small molecule program for Parkinson's disease. Under the agreement, Genentech granted the Company (i) an exclusive, worldwide, sublicensable license under Genentech’s rights to certain patents and patent applications directed to small molecule compounds which bind to and inhibit LRRK2 and (ii) a non-exclusive, worldwide, sublicensable license to certain related know-how, in each case, to develop and commercialize certain compounds and licensed products incorporating any such compound. The Company is obligated to use commercially reasonable efforts during the first three years of the agreement to research, develop and commercialize at least one licensed product. As consideration, the Company paid an upfront fee of $8.5 million and a technology transfer fee of $1.5 million, both of which are recognized as research and development expense for the year ended December 31, 2016. The Company may owe Genentech milestone payments upon the achievement of certain development, regulatory, and commercial milestones, up to a maximum of $315.0 million in the aggregate, as well as royalties on net sales of licensed products ranging from low to high single-digit percentages, with the exact royalty rate dependent on various factors, including (i) whether the compound incorporated in the relevant licensed product is a Genentech-provided compound or a compound acquired or developed by the Company, (ii) the date a compound was first discovered, derived or optimized by the Company, (iii) the existence of patent rights covering the relevant licensed product in the relevant country, (iv) the existence of orphan drug exclusivity covering a licensed product that is a Genentech-provided compound and (v) the level of annual net sales of the relevant licensed product. The Company also has the right to credit a certain amount of its payments of third-party royalty and milestones against royalty and milestones owed to Genentech, up to a maximum reduction of fifty percent. The Company’s royalty payment obligations will expire on a country-by-country and licensed product-by-licensed product basis upon the later of (a) ten years after the first commercial sale of such licensed product in such country and (b) the expiration of the last valid claim of a licensed patent covering such licensed product in such country. Genentech may terminate the agreement if the Company challenges any of the patent rights licensed to the Company by Genentech, or if the Company materially breaches the agreement, subject to specified notice and cure provisions, or enters into bankruptcy or insolvency proceedings. If Genentech terminates the agreement for the Company’s material breach, bankruptcy or insolvency after the Company has made a milestone payment to Genentech, then the Company is obligated to grant to Genentech an exclusive right of first negotiation with respect to certain of the Company’s patents, know-how and regulatory filings directed to Genentech-provided compounds. The Company does not have the right to terminate the agreement without cause, but may terminate the agreement for Genentech’s material breach, subject to specified notice and cure provisions. Unless earlier terminated, the agreement with Genentech will continue in effect until all of the Company’s royalty and milestone payment obligations to Genentech expire. Following expiration of the agreement, the Company will retain the licenses under the intellectual property Genentech licensed to the Company on a non-exclusive, royalty-free basis. The first clinical milestone of $2.5 million became due in June 2017 upon first patient dosing in the Phase 1 clinical trial for DNL201. The full amount was recorded as research and development expense in the year ended December 31, 2017.

Balance Sheet Components

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Balance Sheet Components

Property and Equipment, Net


	December 31,
	2018		2017
	(in thousands)
Lab equipment	$	14,352	$	11,351
Leasehold improvements	22,288		7,737
Computers equipment and purchased software	458		439
Furniture and fixtures	74		65
	37,172		19,592
Less: accumulated depreciation	(12,010)		(4,669)
Total property and equipment, net	$	25,162	$	14,923

Depreciation expense was $7.4 million, $3.1 million and $1.5 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Prepaid Expenses and Other Current Assets


	December 31,
	2018		2017
	(in thousands)
Prepaid research and development expenses	$	6,643	$	946
Collaboration accounts receivable	7,364		—
Prepaid insurance	—		927
Accrued interest on short-term marketable securities	1,182		464

Other prepaid and current assets	1,350		1,044
Total prepaid expenses and other current assets	$	16,539	$	3,381

Other Non-Current Assets


	December 31,
	2018		2017
	(in thousands)
Prepaid research and development expenses	$	5,841	$	—
Other investments	—		500
Restricted cash	1,500		451
Other prepaid and non-current assets	764		490
Total other non-current assets	$	8,105	$	1,441

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Lease Obligations

In September 2015, the Company entered into a non-cancelable operating lease for its former corporate headquarters comprising 38,109 of rentable square feet in a building in South San Francisco (“Former Headquarters Lease”). The Former Headquarters Lease commenced on August 1, 2016 with a lease term of eight years. The Former Headquarters Lease provided for monthly base rent amounts escalating over the term of the lease. In addition, the Former Headquarters Lease provided both a tenant improvement allowance (“TIA”) of up to $7.4 million, of which $1.9 million would be repaid to the landlord in the form of additional monthly rent with interest applied.

In May 2018, the Company entered into an amendment to the Former Headquarters Lease (the "Headquarters Lease Amendment") to relocate and expand its headquarters to 148,020 rentable square feet in a to-be-constructed building located in South San Francisco, California (the "New Premises"). The Headquarters Lease Amendment has a contractual term of ten years from the legal commencement date, which is the later of February 1, 2019 or the date that the premises are ready for occupancy. For accounting purposes, the lease commencement date was determined to be August 1, 2018, which was the date at which the Company obtained control over the property. The Company has an option to extend the lease term for a period of ten years by giving the landlord written notice of the election to exercise the option at least nine months, but not more than twelve months, prior to the expiration of the Headquarters Lease Amendment lease term.

Under the terms of the Former Headquarters Lease, the Company was required to pay a security deposit of $0.5 million, which was increased to $1.5 million under the Headquarters Lease Amendment. This is recorded as other non-current assets in the accompanying consolidated balance sheets.

The Headquarters Lease Amendment provides for monthly base rent amounts escalating over the term of the lease. In addition, the Headquarters Lease Amendment provides a TIA of up to $25.9 million, of which $4.4 million, if utilized, would be repaid to the landlord in the form of additional monthly rent. The Company will also be required to pay its share of operating expenses for the New Premises.

The total $7.4 million TIA under the Former Headquarters Lease was recorded as leasehold improvements and deferred rent liability on the consolidated balance sheet. The Company is amortizing the deferred rent liability as a reduction of rent expense through the expiration of the Headquarters Lease Amendment period, and the leasehold improvement through an increase of depreciation expense of leasehold improvements ratably over the remaining period of expected use.

The portion of the TIA utilized under the Headquarters Lease Amendment was $14.6 million, which is recorded as leasehold improvements and deferred rent liability on the consolidated balance sheet as of December 31, 2018. The Company will amortize the deferred rent liability as a reduction of rent expense and the leasehold improvement as depreciation expense of leasehold improvements ratably over the period of expected use, which is expected to commence in April 2019.

In October 2018, the Company entered into a sublease agreement ("Sublease Agreement") to sublease approximately 36,835 rentable square feet of space in its New Premises. The Sublease Agreement has a term of five years from the commencement date, which is the legal commencement date of the Headquarters Lease Amendment, and provides for the Company to receive monthly base rent amounts escalating over the term of the lease, totaling approximately $14.8 million over the term of the Sublease Agreement.

The Company recognizes rent expense on a straight-line basis over the non-cancelable lease term and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Where leases contain escalation clauses, rent abatements, and/or concessions such as rent holidays and landlord or tenant incentives or allowances, the Company applies them in the determination of straight-line rent expense over the lease term. The Company records tenant improvement allowances as deferred rent and associated expenditures as leasehold improvements that are being amortized over the shorter of their estimated useful life or the term of the lease. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably assured at lease inception.

As of December 31, 2018, the future minimum lease payments under all non-cancellable operating leases are as follows (in thousands):


Year Ending December 31:
2019	$	4,941
2020	9,097
2021	9,716
2022	10,056
2023	10,408
2024 and later	60,882
	$	105,100

Rent expense for the years ended December 31, 2018, 2017 and 2016 was $6.0 million, $2.2 million, $1.0 million, respectively.

Indemnification

In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to vendors, lessors, business partners, board members, officers, and other parties with respect to certain matters, including, but not limited to, losses arising out of breach of such agreements, services to be provided by the Company, negligence or willful misconduct of the Company, violations of law by the Company, or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with directors and certain officers and employees that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. No demands have been made upon the Company to provide indemnification under such agreements, and thus, there are no claims that the Company is aware of that could have a material effect on the Company’s consolidated balance sheets, consolidated statements of operations and comprehensive loss, or consolidated statements of cash flows.

Commitments

Effective September 2017, the Company entered into a Development and Manufacturing Services Agreement as amended (“DMSA”) with Lonza Sales AG (“Lonza”) for the development and manufacture of biologic products. Under the DMSA, the Company will execute purchase orders based on project plans authorizing Lonza to provide development and manufacturing services with respect to certain of the Company's antibody and enzyme products, and will pay for the services provided and batches delivered in accordance with the DMSA and project plan. Unless earlier terminated, the Lonza agreement will expire on September 6, 2022.

As of December 31, 2018 and 2017, the Company had non-cancellable purchase orders for biological product development and manufacturing costs totaling $24.7 million and $0.7 million, respectively. The activities under these purchase orders are expected to be completed by November 2024. During the year ended December 31, 2018, the Company incurred costs of $3.9 million and made payments of $3.4 million for the development and manufacturing services rendered under the agreement. The Company had not incurred any costs or made any payments for the manufacturing services rendered under the agreement for the year ended December 31, 2017. As of December 31, 2018 and 2017, the Company had total non-cancellable purchase commitments of $14.0 million and $0.4 million, respectively, under the DMSA.

In the normal course of business, the Company enters into various firm purchase commitments primarily related to research and development activities. The Company had contractual obligations under license and other agreements of $1.2 million and $1.6 million as of December 31, 2018 and 2017, respectively.

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	Convertible Preferred Stock The Company entered into a preferred stock purchase agreement (“Preferred Stock Purchase Agreement”), with certain investors in May 2015 (the “Initial Closing”), under which the Company agreed to sell up to 45,223,970 shares of Series A-1 convertible preferred stock and 4,361,530 shares of Series A-2 convertible preferred stock. Additionally, at the Initial Closing, the Company concurrently issued 6,295,810 shares of Series A-1 convertible preferred stock for net proceeds of $24.8 million. The Preferred Stock Purchase Agreement provided that, upon Board of Directors approval, each investor would purchase its pro-rata portion of the shares to be issued in one or more additional Series A-1 closings, and in any Series A-2 closings. Further, the Company agreed to sell and issue said shares of Series A-1 convertible preferred stock on the same terms as the first tranche, and to issue said shares of Series A-2 convertible preferred stock on the terms included in the Preferred Stock Purchase Agreement. The second and third Series A-1 closings added further obligations for new investors to participate in the Series A-2 tranches. The Company did not separately account for tranche purchase rights described above as they were not freestanding from the associated shares of convertible preferred stock. In May 2015 (the “First Additional Closing”) the Company and the Series A convertible preferred stock shareholders amended the Preferred Stock Purchase Agreement pursuant to which the Company agreed to sell up to an additional 456,250 shares of Series A-1 convertible preferred stock. Additionally, at the First Additional Closing, the Company issued 3,481,250 shares of Series A-1 convertible preferred stock for net proceeds of $13.9 million and in July 2015 (“Second Additional Closing”), the Company issued an additional 2,420,830 shares of Series A-1 convertible preferred stock for net proceeds of $9.6 million. In January 2016 (the “Third Additional Closing”), the Company issued 500,000 shares of Series A-1 convertible preferred stock, for net proceeds of $2.0 million, and also issued 11,250,000 shares of Series A-1 convertible preferred stock (the “first Tranche Closing”), for net proceeds of $45.0 million. In June 2016 (the “Second Tranche Closing and Series A-2 Closing”), the Company issued 22,166,550 shares of Series A-1 convertible preferred stock and 4,361,530 shares of Series A-2 convertible preferred stock, for net proceeds of $88.7 million and $34.9 million, respectively. All of these shares were sold under the Preferred Stock Purchase Agreement. In June 2016 (the “First Series B-1 Closing”), the Company entered into a preferred stock purchase agreement (“Series B Preferred Stock Purchase Agreement”) with certain investors, under which the Company sold 7,646,240 shares of Series B-1 convertible preferred stock at a price of $16.00 per share for net proceeds of $122.2 million. In connection with this financing, the Company amended and restated its certificate of incorporation to increase the authorized shares of its common stock to 81,787,360 shares and the authorized shares of its preferred stock to 63,288,470 shares, each with a par value of $0.01 per share. The authorized preferred shares consisted of 46,114,433 designated as Series A-1 convertible preferred stock, 4,361,533 designated as Series A-2 convertible preferred stock, 8,125,000 designated Series B-1 convertible preferred stock and 4,687,500 designated Series B-2 convertible preferred stock. In August 2016, (the “Second Series B-1 Closing”), the Company sold 478,120 shares of Series B-1 convertible preferred stock at a price of $16.00 per share for net proceeds of $7.6 million. In November 2017, the Company sold 1,764,710 shares of Series B-2 convertible preferred stock at a price of $17.00 per share for net proceeds of $29.9 million. Prior to the conversion of the convertible preferred stock upon closing of the IPO, the rights, preferences and privileges of the convertible preferred stock were as follows: Dividend Rights The holders of preferred stock were entitled to receive dividends, if and when declared by the Board of Directors, at the rate of $0.08 per share per annum for Series A-1, $0.16 per share per annum for Series A-2, $0.32 per share per annum for Series B-1, and $0.34 per share per annum for Series B-2, from and after the date of issuance of such shares. As of December 31, 2016, and through the date of conversion, no such dividends were declared or accrued. Dividends on any other class of capital stock could not be paid unless the holders of the preferred stock first received, or simultaneously received, the preferred stock dividend. The holders of preferred stock also participated in dividends paid on common stock as if the shares of preferred stock had been converted into shares of common stock and were considered participating securities. Conversion Rights The holders of preferred stock had right to convert at any time into shares of common stock initially at a one-for-one ratio. All shares of the preferred stock would have been automatically converted into shares common stock (i) upon the consent of the holders of at least a majority of the outstanding preferred stock, or (ii) upon the closing of a firmly underwritten initial public offering of common stock at a price of at least $5.00 per share resulting in at least $50.0 million of gross proceeds. The conversion price for each series of preferred stock was subject to an adjustment in the event of stock split, combination, common stock dividend or distribution, reclassification, exchange, substitution, or reorganization. Liquidation Rights In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or a deemed liquidation event, the holders of shares of preferred stock then outstanding were entitled to be paid out of the assets of the Company available for distribution to its stockholders, before any payment could have been made to the holders of common stock, an amount per share equal to the greater of (i) the original issue price for the Series of preferred stock held plus any dividends accrued but unpaid, whether or not declared; or (ii) such amount per share as would have been paid if all shares of preferred stock had been converted to common stock immediately prior to such liquidation, dissolution, winding up or deemed liquidation event. If assets of the Company available were insufficient to pay holders of Preferred Stock the full amount they are entitled to, the holders of preferred stock would have shared ratably in any distribution of the assets available for distribution in proportion to the amounts due such holders. After the payment of all preferential amounts required to be paid to the holders of shares of preferred stock, the remaining assets of the Company would have been distributed among the holders of the shares of common stock, pro rata based on the number of shares held by each such holder. The Company classified its convertible preferred stock outside of stockholders’ deficit as certain change in control events are outside the Company’s control. Redemption Upon certain change in control events that were outside of the Company’s control, including liquidation, sale or transfer of control of the Company, holders of the convertible preferred stock could have caused its redemption. Shares of preferred stock could have been redeemed by the Company at the original issue price for each series of preferred stock plus any dividends accrued but unpaid, whether or not declared, in three annual installments, upon a written request from the holders of a majority of the then outstanding shares of preferred stock, which request could have been made at any time after the fifth anniversary of the Series B-1 original issue date (on or after June 22, 2021). On each of the three annual redemption dates the Company could have redeemed the number of outstanding shares of preferred stock determined by dividing the total number of outstanding shares of preferred stock by the number of remaining redemption dates. Voting Rights Each share of preferred stock had voting rights equal to the number of shares of common stock into which the preferred stock could have been converted immediately after the close of business on the record date. As long as certain investors in Series A convertible preferred stock held 100,000 or more shares of convertible preferred stock purchased pursuant to the Preferred Stock Purchase Agreement, they were entitled to elect individually one member of the Board totaling five Series A Directors. Series B convertible preferred stockholders were entitled to elect one member of the Board by majority vote of the Series B convertible preferred stockholders. Together, Series A and Series B convertible preferred stock investors were entitled to elect two additional members of the Board that were not otherwise an affiliate of the Company or of any investor. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 60,365,020 shares of common stock. As of December 31, 2018 and 2017, the Company did not have any convertible preferred stock issued or outstanding.

Stock-Based Awards

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Awards

2017 Equity Incentive Plan

In December 2017, the Company adopted the 2017 Equity Incentive Plan (the “2017 Plan”), which initially reserved 6,379,238 common shares for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Awards granted under the 2017 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value of the Company's common stock on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.

2015 Stock Incentive Plan

In May 2015, the Company adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which as amended, reserved 8,325,000 common shares for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Awards granted under the 2015 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value of the Company's common stock on the day of grant. For all stock options granted between August 2015 and February 2016 with an exercise price of $0.68, a deemed fair value of $1.20 per share was used in calculating stock-based compensation expense, which was determined using management hindsight. Options granted typically vest over a four-year period but may be granted with different vesting terms.

Upon adoption of the 2017 Plan, no new awards or grants are permitted under the 2015 Plan, and the 169,238 shares that were then unissued and available for future award under the 2015 Plan became available under the 2017 Plan. The 2015 Plan will continue to govern restricted stock awards and option awards previously granted thereunder.

As of December 31, 2018 and 2017, there were 2,289,196 and 6,012,498 common shares available for the Company to grant under the 2017 Plan. As of December 31, 2016, there were 1,813,321 shares available for the Company to grant under the 2015 Plan.

Stock Option Activity

The following table summarizes option award activity under the 2017 Plan and the 2015 Plan:


	Number of Options	Weighted- Average Exercise Price		Weighted- Average remaining contractual life (years)	Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2016	5,374,014	$	1.77	9.03	$	18,873
Options granted	2,183,365	8.78
Options exercised	(695,192)	1.41
Options forfeited	(172,708)	2.21
Balance at December 31, 2017	6,689,479	$	4.08	8.37	$	77,317
Options granted	3,949,778	20.25
Options exercised	(650,471)	2.78
Options forfeited	(376,134)	12.48
Balance at December 31, 2018	9,612,652	$	10.49	8.20	$	97,804
Options vested and expected to vest at December 31, 2018	7,867,920	$	12.66	8.54	$	62,944
Options exercisable at December 31, 2018	1,933,527	$	4.50	7.64	$	31,248

Aggregate intrinsic value represents the difference between the Company’s estimated fair value of its common stock and the exercise price of outstanding options. The total intrinsic value of options exercised was $9.8 million, $3.5 million and $0.7 million for the years ended December 31, 2018, 2017 and 2016, respectively. During the years ended December 31, 2018, 2017, and 2016 the weighted-average grant-date fair value of the options vested was $3.85, $1.94, and $1.36 per share, respectively. The weighted-average grant date fair value of options granted during the years ended December 31, 2018, 2017 and 2016 was $14.79, $6.41, and $2.83 per share, respectively.

Stock Options Granted to Employees with Service-Based Vesting

The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	5.50 - 6.08	6.08	6.00 - 6.08
Volatility	79.4% - 87.4%	81.9% - 91.3%	91.2% - 92.2%
Risk-free interest rate	2.6% - 3.1%	1.8% - 2.3%	1.2% - 2.1%
Dividend yield	—	—	—

Expected Term: The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term).

Expected Volatility: The Company uses an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have sufficient trading history for its common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

Risk-Free Interest Rate: The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant.

Expected Dividend: The Company has not paid and does not anticipate paying any dividends in the near future. Therefore, the expected dividend yield was zero.

Early Exercise of Stock Options

The Company permits early exercise of certain stock options prior to vesting by certain directors and officers. Any shares issued pursuant to unvested options are restricted and subject to repurchase by the Company until the conditions for vesting are met. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported in stockholders’ equity once those shares vest. Upon termination of employment of an option holder, the Company has the right to repurchase, at the original purchase price, any unvested restricted shares.

A total of $0.4 million and $0.1 million was reclassified from other non-current liabilities to stockholders' equity during the year ended December 31, 2018 and 2017, respectively related to vesting of early exercised options. Unvested early exercised options of $0.1 million and $0.5 million remained in other non-current liabilities as of December 31, 2018 and 2017, respectively.

Performance Contingent Stock Options Granted to Employees

In August and November 2015, the Board of Directors granted 250,000 and 500,000 of stock option awards to certain executive officers, respectively. These awards have an exercise price of $0.68 per share.

These awards vest over time and include a performance provision which states that upon the occurrence of a change in control event, the vesting term would accelerate. Through December 31, 2018, the Company continues to believe that the achievement of the requisite performance condition is not probable and, as a result, the expense relating to these grants continues to be recognized over the initial time-based vesting period. If the performance goal is ever deemed to be probable of achievement, the recognition of compensation expense will be accelerated in accordance with the accelerated vesting schedule.

The estimated fair value of employee performance-contingent options was estimated at the date of grant using a Black-Scholes option-pricing model using the same assumptions as the Stock Options Granted to Employees with Service-based Vesting Valuation Assumptions.

Performance and Market Contingent Stock Options Granted to Employees

In August and November 2015, the Board of Directors granted 1,619,738 and 125,000 of performance- and market- contingent awards to members of the senior management team, respectively. These awards have an exercise price of $0.68 per share.

These awards have two separate market triggers for vesting based upon either (i) the successful achievement of stepped target closing prices on a national securities exchange for 90 consecutive trading days later than 180 days after the Company’s initial public offering for its common stock, or (ii) stepped target prices for a change in control transaction. By definition, the market condition in these awards can only be achieved after the performance condition of a liquidity event has been achieved. As such, the requisite service period is based on the estimated period over which the market condition can be achieved. When a performance goal is deemed to be probable of achievement, time-based vesting and recognition of stock-based compensation expense commences. In the event any the milestones are not achieved by the specified timelines, such vesting award will terminate and no longer be exercisable with respect to that portion of the shares. The maximum potential expense associated with the performance- and market- contingent awards is $6.2 million, $5.8 million and $0.4 million of general and administrative and research and development expense, respectively if all of the performance and market conditions are achieved as stated in the option agreement. Through December 31, 2018, the Company continues to believe that the achievement of the requisite performance conditions is not probable and, as a result, no compensation cost has been recognized for these awards.

The Company uses a lattice model with a Monte Carlo simulation to value stock options with performance and market conditions. This valuation methodology utilizes the estimated fair value of the Company’s common stock on grant date and several key assumptions, including expected volatility of the Company’s stock price based on comparable public companies, risk-free rates of return and expected dividend yield.

Stock Options Granted to Non-Employees with Service-Based Vesting Valuation Assumptions

Stock-based compensation related to stock options granted to non-employees is recognized as the stock options are earned. Prior to the adoption of ASU 2018-07 during the third quarter of 2018, the unvested options granted to non-employees were revalued using the Company's estimate of fair value on each reporting date. Subsequent to the adoption of ASU 2018-07, existing stock options granted to non-employees will no longer be revalued, and the estimated fair value of new stock options granted to non-employees will be calculated on the date of grant and not remeasured, similar to stock options granted to employees.

The estimated fair value of the stock options granted to non-employees has been calculated using the Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	7.14 - 9.86	7.50 - 8.09	8.50 - 9.70
Volatility	88.9% - 103.1%	86.1% - 88.1%	95.3% - 98.2%
Risk-free interest rate	2.7% - 3.0%	2.3% - 2.4%	2.4%
Dividend yield	—	—	—

Restricted Stock Activity

Under the 2017 Plan, the Company may grant restricted stock awards ("RSAs"), which represent restricted shares of common stock issued upon the date of grant in which the recipient's rights in the stock are restricted until the shares are vested, and restricted stock units ("RSUs"), which represent a commitment to issue shares of common stock in the future upon vesting. The fair value of restricted stock underlying the RSAs and RSUs is determined based on the closing market price of the Company's common stock on the date of grant.

Aggregated information regarding RSAs and RSUs granted under the Plan for the years ended December 31, 2018 and 2017 is summarized below:


	Share Awards & Units	Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2016	3,922,638	$	0.18
Granted	—	—
Vested	(1,628,850)	0.18
Forfeited	—	—
Unvested at December 31, 2017	2,293,788	$	0.18
Granted	149,658	15.92
Vested	(1,940,203)	0.18
Forfeited	—	—
Unvested at December 31, 2018	503,243	$	4.86
Vested and expected to vest – December 31, 2018	503,243	$	4.86

There was $2.0 million and $0.3 million of total unrecognized stock-based compensation related to unvested RSAs and RSUs at December 31, 2018 and 2017 respectively, all of which is expected to be recognized over a remaining weighted-average period of 0.6 years and 1.0 year, respectively.

Employee Stock Purchase Plan

In December 2017, the Company adopted the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which initially reserved 1,000,000 shares of common stock for employee purchases under terms and provisions established by the Board of Directors. Under the 2017 ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the exercise date. The 2017 ESPP provides for consecutive, overlapping 12-month offering periods. The offering periods are scheduled to start on the first trading day on or after May 31 or November 30 of each year, except for the first offering period which commenced on December 8, 2017, the first trading day after the effective date of the Company’s registration statement. Contributions under the 2017 ESPP are limited to a maximum of 15% of an employee's eligible compensation.

The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Year Ended December 31,
	2018	2017
Expected term (in years)	0.50 - 1.00	0.48 - 0.98
Volatility	61.0% - 64.3%	50.0% - 51.3%
Risk-free interest rate	2.1% - 2.7%	1.5% - 1.7%
Dividend yield	—%	—%

Stock-Based Compensation Expense

The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development	$	10,093	$	2,852	$	2,078
General and administrative	8,698		1,557		873
Total	$	18,791	$	4,409	$	2,951

As of December 31, 2018, total unamortized stock-based compensation expense related to unvested employee and non-employee awards that are expected to vest was $57.6 million. The weighted-average period over which such stock-based compensation expense will be recognized is approximately 3.0 years.

The Company recorded stock-based compensation expense for options issued to non-employees of $0.7 million, $0.7 million, and $1.0 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Defined Contribution Plan	12 Months Ended
Defined Contribution Plan	Dec. 31, 2018
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution PlanIn January 2017, the Company began to sponsor a 401(k) retirement savings plan for the benefit of its employees, including Denali's named executive officers, who satisfy certain eligibility requirements. Under the 401(k) plan, eligible employees may elect to defer a portion of their compensation, within the limits prescribed by the Code, on a pre-tax or after-tax (Roth) basis through contributions to the 401(k) plan. The 401(k) plan authorizes employer safe harbor contributions. The 401(k) plan is intended to qualify under Sections 401(a) and 501(a) of the Code. As a tax-qualified retirement plan, pre-tax contributions to the 401(k) plan and earnings on those pre-tax contributions are not taxable to the employees until distributed from the 401(k) plan, and earnings on Roth contributions are not taxable when distributed from the 401(k) plan. The Company made contributions to the Plan for eligible participants, and recorded contribution expenses of $0.9 million and $0.5 million for the years ended December 31, 2018 and 2017, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

The Company has reported pre-tax operating losses for all the periods presented. The Company has not reflected any benefit for corresponding tax net operating loss carryforwards in the accompanying consolidated financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.

The effective tax rate for the years ended December 31, 2018, 2017, and 2016 is different from the federal statutory tax rate primarily due to the valuation allowance against deferred tax assets. The Company's effective tax rate differs from the federal statutory rate as follows:


	Year Ended December 31,
	2018		2017		2016
Taxes at the U.S. statutory tax rate	21.0	%	34.0	%	34.0	%
Effect of Tax Act	—		(26.6)		—
Change in valuation allowance	(20.7)		(7.3)		(32.0)
Contingent consideration issued in tax-free reorganization	—		—		(2.1)
Research tax credits	10.2		1.0		0.6
Stock-based compensation	(0.7)		(1.0)		(0.5)
Nondeductible acquisition-related costs	(9.5)		—		—
Other	(0.3)		(0.1)		—
Total provision for income taxes	—	%	—	%	—	%

The components of the Company’s net deferred tax assets are as follows (in thousands):


	December 31,
	2018		2017
Deferred tax assets:
Net operating loss carryforwards	$	35,023	$	37,606
Tax credit carryforwards	9,565		4,072
Reserves and accruals	7,309		3,328
Capitalized start-up costs	4,464		6,248
Intangibles	5,401		5,300
Share based compensation	3,305		733
Other	7		7
Gross deferred tax assets	65,074		57,294
Valuation allowance	(60,915)		(54,650)
Net deferred tax assets	4,159		2,644
Deferred tax liabilities:
Property and equipment	(4,159)		(2,618)
Stock-based compensation	—		(26)
Net deferred tax assets	$	—	$	—

Recognition of deferred tax assets is appropriate when realization of such assets is more likely than not. Based upon the weight of available evidence, especially the uncertainties surrounding the realization of deferred tax assets through future taxable income, the Company believes it is not more likely than not that the deferred tax assets will be fully realizable. Accordingly, the Company has provided a 100% valuation allowance against its net deferred tax assets as of December 31, 2018 and 2017.

On December 22, 2017, the U.S. government enacted the Tax Act. The Act reduced the corporate tax rate to 21 percent, effective January 1, 2018. The tax rate decrease resulted in a reduction of $6.3 million in the Company's deferred tax assets as of December 31, 2017, and a corresponding decrease of the same amount in the valuation allowance, as substantially all of the Company's deferred tax assets, net of deferred tax liabilities, are subject to a full valuation allowance.

As of December 31, 2018, the Company has federal net operating loss (“NOL”) carryforwards of approximately $118.6 million, which are available to reduce future taxable income, and has federal tax credits of approximately $7.5 million which may be used to offset future tax liabilities. The federal NOL and federal tax credit carryforwards will begin to expire in 2035. The Company also has state NOL carryforwards of approximately $134.0 million, which are available to reduce future taxable income, and has state tax credits of approximately $5.7 million which may be used to offset future tax liabilities. The state NOL will begin to expire in 2035 and the state tax credit carryforwards will be carried forward indefinitely. The NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service (“IRS”) and state tax authorities and may become subject to an annual limitation in the event of certain future cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. Annual limitations may result in expiration of net operating loss and tax credit carryforwards before some or all of such amounts have been utilized.

The Company follows the provisions of ASC 740, Accounting for Income Taxes, and the accounting guidance related to accounting for uncertainty in income taxes. The Company determines its uncertain tax positions based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings is more likely than not to be sustained upon examination by the relevant income tax authorities.

A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):


	December 31,
	2018		2017		2016
Unrecognized tax benefits at January 1	$	1,146	$	531	$	122
Additions for tax positions taken in a prior year	—		—		7
Additions for tax positions taken in the current year	1,506		640		411
Reductions for tax positions taken in the prior year	(10)		(25)		(9)
Unrecognized tax benefits at December 31	$	2,642	$	1,146	$	531

If recognized, none of the unrecognized tax benefits would reduce the annual effective tax rate, primarily due to corresponding adjustments to the valuation allowance. The Company will recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. As of December 31, 2018, no liability has been recorded for potential interest or penalties. The Company does not expect the unrecognized tax benefits to change significantly over the next 12 months.

Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Net Loss and Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss and Net Loss Per Share

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,
	2018		2017		2016
Numerator:
Net loss	$	(36,240)	$	(88,185)	$	(86,652)
Denominator:
Weighted average common shares outstanding	92,621,991		14,964,144		6,424,720
Net loss per share, basic and diluted	$	(0.39)	$	(5.89)	$	(13.49)

Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive securities would have been anti-dilutive. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:


	Year Ended December 31,
	2018	2017	2016
Series A-1 convertible preferred stock	—	—	46,114,423
Series A-2 convertible preferred stock	—	—	4,361,527
Series B-1 convertible preferred stock	—	—	8,124,365
Options issued and outstanding and ESPP shares issuable and outstanding	9,789,594	6,835,313	5,374,014
Restricted shares subject to future vesting	503,243	2,293,788	3,922,638
Early exercised common stock subject to future vesting	135,424	369,796	510,417
Shares to be issued under Incro acquisition agreement	—	—	81,164
Total	10,428,261	9,498,897	68,488,548

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

The following table provides the selected quarterly financial data for the years ended December 31, 2018 and 2017 (in thousands, except per share amounts):


	Quarter Ended
	December 31, 2018		September 30, 2018		June 30, 2018		March 31, 2018
Collaboration revenue	$	125,676	$	1,195	$	1,648	$	641
Profit (loss) from operations	74,722		(37,964)		(57,382)		(25,748)
Net income (loss)	77,533		(35,371)		(54,724)		(23,678)
Net income (loss) per share, basic	$	0.82	$	(0.38)	$	(0.59)	$	(0.26)
Net income (loss) per share, diluted⁽¹⁾	$	0.79	$	(0.38)	$	(0.59)	$	(0.26)

_________________________________________________

⁽¹⁾Diluted net income per share for the quarter ended December 31, 2018 is calculated using 97,746,224 shares, which is computed by giving effect to all dilutive potential ordinary shares including options.


	Quarter Ended
	December 31, 2017		September 30, 2017		June 30, 2017		March 31, 2017
Loss from operations	$	(23,540)	$	(22,288)	$	(22,568)	$	(21,744)
Net loss	(22,887)		(21,844)		(22,134)		(21,320)
Net loss per share, basic and diluted	$	(0.74)	$	(2.14)	$	(2.29)	$	(2.36)

Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Organization and Description of Business

Basis of Presentation

Basis of PresentationThese consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”).

Reverse Stock Split

Initial Public Offering

Principles of Consolidation

Use of Estimates

Concentration of Credit Risk and Other Risks and Uncertainties

Segments

Fair Value of Financial Instruments

Level 1 – inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities.

Level 2 – inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

Cash and Cash Equivalents

The Company considers all highly liquid investments with original maturities of 90 days or less at the date of purchase to be cash and cash equivalents. Cash equivalents are stated at fair value.

Marketable Securities

Restricted Cash

Accounts receivable

Derivatives and Hedging Activities

Property and Equipment, Net


Leasehold improvements			Shorter of life of asset or lease term
Manufacturing and laboratory equipment			five years
Computer hardware and software			three years
Office furniture and equipment			five years

Impairment of Long-Lived Assets

Deferred Rent

Revenue Recognition

License and Collaboration Revenue

Research and Development Expenses

Stock-Based Compensation

Income Taxes

The Company’s policy is to recognize interest and penalties accrued on any unrecognized tax benefits as a component of income tax expense.

Comprehensive Loss

Net Income (Loss) per Share

Recently Issued and Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Estimated Useful Life

Depreciation is computed using the straight-line method over the related estimated useful lives as presented in the table below. Significant additions and improvements are capitalized, while repairs and maintenance are charged to expense as incurred.


Leasehold improvements			Shorter of life of asset or lease term
Manufacturing and laboratory equipment			five years
Computer hardware and software			three years
Office furniture and equipment			five years

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Assets and liabilities measured at fair value

Assets and liabilities measured at fair value at each balance sheet date are as follows (in thousands):


	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	42,225	$	—	$	—	$	42,225
U.S. government treasuries	1,499		—		—		1,499
Commercial paper	—		9,979		—		9,979
Short-term marketable securities:
U.S. government treasuries	219,754		—		—		219,754
U.S. government agency securities	—		73,151		—		73,151
Corporate debt securities	—		71,675		—		71,675
Commercial paper	—		22,594		—		22,594
Long-term marketable securities:
U.S. government treasuries	117,131		—		—		117,131
U.S. government agency securities	—		1,977		—		1,977
Corporate debt securities	—		28,773		—		28,773
Foreign currency derivative contracts	—		14		—		14
Total	$	380,609	$	208,163	$	—	$	588,772
Liabilities:
Foreign currency derivative contracts	$	—	$	182	$	—	$	182
Total	$	—	$	182	$	—	$	182


	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents
Money market funds	$	212,868	$	—	$	—	$	212,868
Short-term marketable securities:
U.S. government treasuries	42,587		—		—		42,587
U.S. government agency securities	—		106,139		—		106,139
Corporate debt securities	—		39,125		—		39,125
Long-term marketable securities:
U.S. government treasuries	39,848		—		—		39,848
U.S. government agency securities	—		19,911		—		19,911
Corporate debt securities	—		991		—		991
Total	$	295,303	$	166,166	$	—	$	461,469

Cash and Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2018

Cash and Cash Equivalents [Abstract]

Schedule of Restricted Cash and Cash Equivalents


	December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	77,123	$	218,375	$	39,853
Restricted cash included within prepaid expenses and other current assets	—		84		—
Restricted cash included within other non-current assets	1,500		451		535
Total cash, cash equivalents, and restricted cash	$	78,623	$	218,910	$	40,388

Summary of Available for Sale Securities

The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands):


	December 31, 2018
	Amortized Cost		Unrealized Holding Gains		Unrealized Holding Losses		Aggregate Fair Value
Short-term marketable securities:
U.S. government treasuries	$	220,081	$	29	$	(356)	$	219,754
U.S. government agency securities	73,373		—		(222)		73,151
Corporate debt securities	71,940		1		(266)		71,675

Commercial paper	22,594		—		—		22,594
Total short-term marketable securities	387,988		30		(844)		387,174
Long-term marketable securities:
U.S. government treasuries	116,878		329		(76)		117,131
U.S. government agency securities	1,975		2		—		1,977
Corporate debt securities	28,864		8		(99)		28,773
Total long-term marketable securities	147,717		339		(175)		147,881
Total	$	535,705	$	369	$	(1,019)	$	535,055


	December 31, 2017
	Amortized Cost		Unrealized Holding Gains		Unrealized Holding Losses		Aggregate Fair Value
Short-term marketable securities:
U.S. government treasuries	$	42,614	$	—	$	(27)	$	42,587
U.S. government agency securities	106,368		—		(229)		106,139
Corporate debt securities	39,197		—		(72)		39,125
Total short-term marketable securities	188,179		—		(328)		187,851
Long-term marketable securities:
U.S. government treasuries	39,868		—		(20)		39,848
U.S. government agency securities	19,931		—		(20)		19,911
Corporate debt securities	991		—		—		991
Total long-term marketable securities	60,790		—		(40)		60,750
Total	$	248,969	$	—	$	(368)	$	248,601

Derivative Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Summary of Forward Foreign Currency Exchange Contracts Outstanding

The following table summarizes the Company’s forward foreign currency exchange contracts outstanding as of December 31, 2018 (notional amounts in thousands):

Foreign Exchange Contracts

Number of Contracts

Aggregate Notional⁽¹⁾ Amount in Foreign Currency

Maturity

Euros

2,343

Jan. 2019 - Nov. 2019

British Pounds

4,017

Jan. 2019 - Nov. 2019

Swiss Francs

1,100

Jan. 2019 - Nov. 2019

Total

_________________________________________________

⁽¹⁾The notional amount represents the net amount of foreign currency that will be received upon maturity of the forward contracts.

- Definition

Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

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+ Details

Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Summary of Significant Changes in the Net Contract Liability Balance

Significant changes in the net contract liability balance during the year ended December 31, 2018 are as follows (in thousands):


	Contract liability
Balance at January 1, 2018	$	—
Increases due to cash received, excluding amounts recognized as revenue during the period	66,619
Decreases due to revenue recognized in the period for which cash has not been received	(1,701)
Balance at December 31, 2018	$	64,918

Summary of Collaboration Revenue

Revenue disaggregated by collaboration agreement and performance obligation for the year ended December 31, 2018 is as follows (in thousands):


	Revenue
Takeda Collaboration Agreement	$	5,677
Sanofi Collaboration Agreement:
CNS program license	73,932
Peripheral program license	47,148
Alzheimer's Disease Services	60
Retained Activities	2,343
Total Sanofi Collaboration Revenue	123,483
Total Collaboration Revenue	$	129,160

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Property and Equipment, Net


	December 31,
	2018		2017
	(in thousands)
Lab equipment	$	14,352	$	11,351
Leasehold improvements	22,288		7,737
Computers equipment and purchased software	458		439
Furniture and fixtures	74		65
	37,172		19,592
Less: accumulated depreciation	(12,010)		(4,669)
Total property and equipment, net	$	25,162	$	14,923

Prepaid Expenses and Other Current Assets


	December 31,
	2018		2017
	(in thousands)
Prepaid research and development expenses	$	6,643	$	946
Collaboration accounts receivable	7,364		—
Prepaid insurance	—		927
Accrued interest on short-term marketable securities	1,182		464

Other prepaid and current assets	1,350		1,044
Total prepaid expenses and other current assets	$	16,539	$	3,381

Other Non-Current Assets


	December 31,
	2018		2017
	(in thousands)
Prepaid research and development expenses	$	5,841	$	—
Other investments	—		500
Restricted cash	1,500		451
Other prepaid and non-current assets	764		490
Total other non-current assets	$	8,105	$	1,441

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Summary of Company's Future Minimum Lease Commitments

As of December 31, 2018, the future minimum lease payments under all non-cancellable operating leases are as follows (in thousands):


Year Ending December 31:
2019	$	4,941
2020	9,097
2021	9,716
2022	10,056
2023	10,408
2024 and later	60,882
	$	105,100

Stock-Based Awards (Tables)

12 Months Ended

Dec. 31, 2018

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Stock Option Activity

The following table summarizes option award activity under the 2017 Plan and the 2015 Plan:


	Number of Options	Weighted- Average Exercise Price		Weighted- Average remaining contractual life (years)	Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2016	5,374,014	$	1.77	9.03	$	18,873
Options granted	2,183,365	8.78
Options exercised	(695,192)	1.41
Options forfeited	(172,708)	2.21
Balance at December 31, 2017	6,689,479	$	4.08	8.37	$	77,317
Options granted	3,949,778	20.25
Options exercised	(650,471)	2.78
Options forfeited	(376,134)	12.48
Balance at December 31, 2018	9,612,652	$	10.49	8.20	$	97,804
Options vested and expected to vest at December 31, 2018	7,867,920	$	12.66	8.54	$	62,944
Options exercisable at December 31, 2018	1,933,527	$	4.50	7.64	$	31,248

Summary of Assumptions Used for Estimating the Fair Value of Stock Granted

Stock Options Granted to Employees with Service-Based Vesting

The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	5.50 - 6.08	6.08	6.00 - 6.08
Volatility	79.4% - 87.4%	81.9% - 91.3%	91.2% - 92.2%
Risk-free interest rate	2.6% - 3.1%	1.8% - 2.3%	1.2% - 2.1%
Dividend yield	—	—	—

Restricted Stock Activity

Aggregated information regarding RSAs and RSUs granted under the Plan for the years ended December 31, 2018 and 2017 is summarized below:


	Share Awards & Units	Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2016	3,922,638	$	0.18
Granted	—	—
Vested	(1,628,850)	0.18
Forfeited	—	—
Unvested at December 31, 2017	2,293,788	$	0.18
Granted	149,658	15.92
Vested	(1,940,203)	0.18
Forfeited	—	—
Unvested at December 31, 2018	503,243	$	4.86
Vested and expected to vest – December 31, 2018	503,243	$	4.86

Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted in ESPP

The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Year Ended December 31,
	2018	2017
Expected term (in years)	0.50 - 1.00	0.48 - 0.98
Volatility	61.0% - 64.3%	50.0% - 51.3%
Risk-free interest rate	2.1% - 2.7%	1.5% - 1.7%
Dividend yield	—%	—%

Summary of Stock-Based Compensation Expense

The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development	$	10,093	$	2,852	$	2,078
General and administrative	8,698		1,557		873
Total	$	18,791	$	4,409	$	2,951

Non-employees

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Assumptions Used for Estimating the Fair Value of Stock Granted

The estimated fair value of the stock options granted to non-employees has been calculated using the Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	7.14 - 9.86	7.50 - 8.09	8.50 - 9.70
Volatility	88.9% - 103.1%	86.1% - 88.1%	95.3% - 98.2%
Risk-free interest rate	2.7% - 3.0%	2.3% - 2.4%	2.4%
Dividend yield	—	—	—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Tax Rate Reconciliation

The Company's effective tax rate differs from the federal statutory rate as follows:


	Year Ended December 31,
	2018		2017		2016
Taxes at the U.S. statutory tax rate	21.0	%	34.0	%	34.0	%
Effect of Tax Act	—		(26.6)		—
Change in valuation allowance	(20.7)		(7.3)		(32.0)
Contingent consideration issued in tax-free reorganization	—		—		(2.1)
Research tax credits	10.2		1.0		0.6
Stock-based compensation	(0.7)		(1.0)		(0.5)
Nondeductible acquisition-related costs	(9.5)		—		—
Other	(0.3)		(0.1)		—
Total provision for income taxes	—	%	—	%	—	%

Schedule of Deferred Tax Assets and Liabilities

The components of the Company’s net deferred tax assets are as follows (in thousands):


	December 31,
	2018		2017
Deferred tax assets:
Net operating loss carryforwards	$	35,023	$	37,606
Tax credit carryforwards	9,565		4,072
Reserves and accruals	7,309		3,328
Capitalized start-up costs	4,464		6,248
Intangibles	5,401		5,300
Share based compensation	3,305		733
Other	7		7
Gross deferred tax assets	65,074		57,294
Valuation allowance	(60,915)		(54,650)
Net deferred tax assets	4,159		2,644
Deferred tax liabilities:
Property and equipment	(4,159)		(2,618)
Stock-based compensation	—		(26)
Net deferred tax assets	$	—	$	—

Schedule of Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):


	December 31,
	2018		2017		2016
Unrecognized tax benefits at January 1	$	1,146	$	531	$	122
Additions for tax positions taken in a prior year	—		—		7
Additions for tax positions taken in the current year	1,506		640		411
Reductions for tax positions taken in the prior year	(10)		(25)		(9)
Unrecognized tax benefits at December 31	$	2,642	$	1,146	$	531

Net Loss and Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Summary of Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,
	2018		2017		2016
Numerator:
Net loss	$	(36,240)	$	(88,185)	$	(86,652)
Denominator:
Weighted average common shares outstanding	92,621,991		14,964,144		6,424,720
Net loss per share, basic and diluted	$	(0.39)	$	(5.89)	$	(13.49)

Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations

Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:


	Year Ended December 31,
	2018	2017	2016
Series A-1 convertible preferred stock	—	—	46,114,423
Series A-2 convertible preferred stock	—	—	4,361,527
Series B-1 convertible preferred stock	—	—	8,124,365
Options issued and outstanding and ESPP shares issuable and outstanding	9,789,594	6,835,313	5,374,014
Restricted shares subject to future vesting	503,243	2,293,788	3,922,638
Early exercised common stock subject to future vesting	135,424	369,796	510,417
Shares to be issued under Incro acquisition agreement	—	—	81,164
Total	10,428,261	9,498,897	68,488,548

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Consolidated Quarterly Financial Information (Unaudited)

The following table provides the selected quarterly financial data for the years ended December 31, 2018 and 2017 (in thousands, except per share amounts):


	Quarter Ended
	December 31, 2018		September 30, 2018		June 30, 2018		March 31, 2018
Collaboration revenue	$	125,676	$	1,195	$	1,648	$	641
Profit (loss) from operations	74,722		(37,964)		(57,382)		(25,748)
Net income (loss)	77,533		(35,371)		(54,724)		(23,678)
Net income (loss) per share, basic	$	0.82	$	(0.38)	$	(0.59)	$	(0.26)
Net income (loss) per share, diluted⁽¹⁾	$	0.79	$	(0.38)	$	(0.59)	$	(0.26)

_________________________________________________


	Quarter Ended
	December 31, 2017		September 30, 2017		June 30, 2017		March 31, 2017
Loss from operations	$	(23,540)	$	(22,288)	$	(22,568)	$	(21,744)
Net loss	(22,887)		(21,844)		(22,134)		(21,320)
Net loss per share, basic and diluted	$	(0.74)	$	(2.14)	$	(2.29)	$	(2.36)

Significant Accounting Policies - Narrative (Details)		1 Months Ended	12 Months Ended
Significant Accounting Policies - Narrative (Details)	Dec. 07, 2017 USD ($) $ / shares shares	Nov. 30, 2017 shares	Dec. 31, 2018 USD ($) derivative_instrument shares	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($) shares
Significant Accounting Policies [Line Items]
Common stock, shares authorized (in shares)	400,000,000		400,000,000		400,000,000
Convertible preferred stock, shares authorized (in shares)	40,000,000		40,000,000		40,000,000
Off-balance sheet concentrations of credit risk \| $			$ 0		$ 0
Number of operating segments \| derivative_instrument			1
IPO
Significant Accounting Policies [Line Items]
Shares sold during initial public offering (shares)	15,972,221
Share price (usd per share) \| $ / shares	$ 18.00
Proceeds from issuance of common stock, net of underwriting discounts and commissions and offering expenses \| $	$ 264,300,000
Convertible preferred stock converted into shares of common stock (in shares)	60,365,020
Amount reclassified to common stock and additional paid-in capital \| $	$ 378,600,000
Over-Allotment Option
Significant Accounting Policies [Line Items]
Shares sold during initial public offering (shares)	2,083,333
Common Stock
Significant Accounting Policies [Line Items]
Reverse stock split (in shares)		4
Convertible Preferred Stock
Significant Accounting Policies [Line Items]
Reverse stock split (in shares)		4
Accounting Standards Update 2016-02 \| Pro Forma
Significant Accounting Policies [Line Items]
Lease liabilities \| $				$ 46,100,000
Right of use asset \| $				$ 46,100,000

Significant Accounting Policies - Property and Equipment, Net (Details)	12 Months Ended
	Dec. 31, 2018
Lab equipment
Property, Plant and Equipment [Line Items]
Useful life term (in years)	5 years
Computers equipment and purchased software
Property, Plant and Equipment [Line Items]
Useful life term (in years)	3 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Useful life term (in years)	5 years

Fair Value Measurements (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	$ 535,055,000	$ 248,601,000
Asset - foreign currency derivative contracts	13,669
Total asset fair value measurements	588,772,000	461,469,000
Liability - foreign currency derivative contracts	182,000
Total liability fair value measurements	182,000
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	42,225,000	212,868,000
U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,499,000
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,979,000
Short-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	219,754,000	42,587,000
Short-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	73,151,000	106,139,000
Short-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	71,675,000	39,125,000
Short-term marketable securities: \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	22,594,000
Long-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	117,131,000	39,848,000
Long-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	1,977,000	19,911,000
Long-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	28,773,000	991,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	0
Total asset fair value measurements	380,609,000	295,303,000
Liability - foreign currency derivative contracts	0
Total liability fair value measurements	0
Level 1 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	42,225,000	212,868,000
Level 1 \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,499,000
Level 1 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 1 \| Short-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	219,754,000	42,587,000
Level 1 \| Short-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 1 \| Short-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 1 \| Short-term marketable securities: \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0
Level 1 \| Long-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	117,131,000	39,848,000
Level 1 \| Long-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 1 \| Long-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	14,000
Total asset fair value measurements	208,163,000	166,166,000
Liability - foreign currency derivative contracts	182,000
Total liability fair value measurements	182,000
Level 2 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 2 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,979,000
Level 2 \| Short-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 2 \| Short-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	73,151,000	106,139,000
Level 2 \| Short-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	71,675,000	39,125,000
Level 2 \| Short-term marketable securities: \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	22,594,000
Level 2 \| Long-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 2 \| Long-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	1,977,000	19,911,000
Level 2 \| Long-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	28,773,000	991,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	0
Total asset fair value measurements	0	0
Liability - foreign currency derivative contracts	0
Total liability fair value measurements	0
Level 3 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3 \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 3 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 3 \| Short-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 3 \| Short-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 3 \| Short-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 3 \| Short-term marketable securities: \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0
Level 3 \| Long-term marketable securities: \| U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 3 \| Long-term marketable securities: \| U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	0	0
Level 3 \| Long-term marketable securities: \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair Value	$ 0	$ 0

Cash and Marketable Securities - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 77,123	$ 218,375	$ 39,853
Restricted cash included within prepaid expenses and other current assets	0	84	0
Restricted cash included within other non-current assets	1,500	451	535
Total cash, cash equivalents, and restricted cash	$ 78,623	$ 218,910	$ 40,388	$ 30,824

Cash and Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 535,705	$ 248,969
Unrealized Holding Gains	369	0
Unrealized Holding Losses	(1,019)	(368)
Aggregate Fair Value	535,055	248,601
Short-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	387,988	188,179
Unrealized Holding Gains	30	0
Unrealized Holding Losses	(844)	(328)
Aggregate Fair Value	387,174	187,851
Short-term marketable securities: \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	220,081	42,614
Unrealized Holding Gains	29	0
Unrealized Holding Losses	(356)	(27)
Aggregate Fair Value	219,754	42,587
Short-term marketable securities: \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,373	106,368
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(222)	(229)
Aggregate Fair Value	73,151	106,139
Short-term marketable securities: \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	71,940	39,197
Unrealized Holding Gains	1	0
Unrealized Holding Losses	(266)	(72)
Aggregate Fair Value	71,675	39,125
Short-term marketable securities: \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,594
Unrealized Holding Gains	0
Unrealized Holding Losses	0
Aggregate Fair Value	22,594
Long-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	147,717	60,790
Unrealized Holding Gains	339	0
Unrealized Holding Losses	(175)	(40)
Aggregate Fair Value	147,881	60,750
Long-term marketable securities: \| U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	116,878	39,868
Unrealized Holding Gains	329	0
Unrealized Holding Losses	(76)	(20)
Aggregate Fair Value	117,131	39,848
Long-term marketable securities: \| U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,975	19,931
Unrealized Holding Gains	2	0
Unrealized Holding Losses	0	(20)
Aggregate Fair Value	1,977	19,911
Long-term marketable securities: \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,864	991
Unrealized Holding Gains	8	0
Unrealized Holding Losses	(99)	0
Aggregate Fair Value	$ 28,773	$ 991

Cash and Marketable Securities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Other-than-temporary impairment	$ 0	$ 0	$ 0

Derivative Financial Instruments - Summary of Forward Foreign Currency Exchange Contracts Outstanding (Details) € in Thousands, £ in Thousands, SFr in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) derivative_instrument	Dec. 31, 2018 EUR (€) derivative_instrument	Dec. 31, 2018 GBP (£) derivative_instrument	Dec. 31, 2018 CHF (SFr) derivative_instrument
Derivative [Line Items]
Derivative liability \| $	$ 182,000
Derivative asset \| $	13,669
Net loss on derivative instruments \| $	$ (200,000)
Foreign Exchange Forward - Euros \| Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts	23	23	23	23
Aggregate Notional Amount in Foreign Currency \| €		€ 2,343
Foreign Exchange Forward - British Pounds \| Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts	25	25	25	25
Aggregate Notional Amount in Foreign Currency \| £			£ 4,017
Foreign Exchange Forward - Swiss Francs \| Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts	20	20	20	20
Aggregate Notional Amount in Foreign Currency \| SFr				SFr 1,100
Foreign Exchange Contracts \| Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts	68	68	68	68

Acquisition (Details) $ in Thousands			12 Months Ended
Acquisition (Details) $ in Thousands	May 30, 2018 USD ($) derivative_instrument	Aug. 24, 2016 USD ($) derivative_instrument	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense			$ 143,183	$ 74,460	$ 75,702
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront option fee			500
Estimated net liabilities	$ 200
Number of Fcab targets \| derivative_instrument		3
Fcab target period		3 years
Upfront fee paid		$ 5,500
Additional Fcab targets \| derivative_instrument	2
Additional Fcab target obligation	$ 6,000
Maximum development plan period for research costs		24 months
Research and development expense			700	$ 1,100	$ 300
Upfront purchase price less estimated net liabilities acquired			$ 17,800
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma \| Exercise of buy-out option
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial option exercise payments	18,000
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma \| Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense		$ 2,100
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma \| Maximum \| Exercise of buy-out option
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones	$ 447,000

Collaboration Agreements - Sanofi (Details) - Collaborative Arrangement - USD ($) $ in Millions	Nov. 20, 2018	Dec. 31, 2018	Oct. 29, 2018
Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment	$ 125.0
Milestone payments upon achievement of certain clinical, regulatory and sales milestone events	1,100.0
Funded percentage			70.00%
Receivable		$ 2.3
Sanofi \| CNS Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical and regulatory milestone payments	600.0
Sanofi \| Peripheral Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical, regulatory and commercial milestone payments	$ 495.0
Denali
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Funded percentage			30.00%

Collaboration Agreements - Sanofi Performance Obligation (Details) $ in Millions	Dec. 31, 2018 USD ($)
Alzheimer's Disease Services \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation	$ 3.9

Collaboration Agreements - Takeda (Details) $ / shares in Units, $ in Thousands			3 Months Ended				12 Months Ended
	Feb. 23, 2018 USD ($) program $ / shares shares	Jan. 03, 2018 USD ($) program	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales			$ 125,676	$ 1,195	$ 1,648	$ 641	$ 129,160	$ 0	$ 0
Collaborative Arrangement \| Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Target option period		5 years
Upfront payment	$ 40,000	$ 40,000
Preclinical milestone payment received	5,000
Option fee		$ 5,000
Number of collaboration programs with cost-sharing, minimum \| program		2
Variable consideration relating to future milestones	26,000
Preclinical milestone payment earned not yet received			5,000				5,000			$ 5,000
Remaining preclinical milestones, cost and profit sharing income, and the development and commercial milestones	$ 44,000
Number of performance obligations \| program	3
Contract liability			$ 64,918				64,918	$ 0
Revenue recognized							(1,701)
Milestones recognized							15,000
Collaborative Arrangement \| Takeda Pharmaceutical Company Limited \| Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales							$ 0
Collaborative Arrangement \| Takeda Pharmaceutical Company Limited \| United States
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of collaboration programs the Company will lead \| program		1
Collaborative Arrangement \| Share Purchase Agreement \| Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common stock (in shares) \| shares	4,214,559
Purchase price	$ 110,000
Fair market value of common stock	$ 94,400
Closing stock price (usd per share) \| $ / shares	$ 22.40
Premium on sale of common stock	$ 15,600
Collaborative Arrangement \| Maximum \| Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preclinical milestone payments per program		$ 25,000
Total aggregate payments due upon achievement of certain preclinical milestone events		75,000
Aggregated option exercise fee		15,000
Total aggregate payments due upon achievement of certain clinical and regulatory milestone events		707,500
Milestone payments per biologic product upon achievement of a certain sales-based milestone		75,000
Milestone payments upon achievement of biologic product from each program		$ 225,000

Collaboration Agreements - Summary of Significant Changes in Net Contract Liability Balance (Details) - Collaborative Arrangement - Takeda Pharmaceutical Company Limited $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Change In Contract with Customer, Liability [Roll Forward]
Balance at January 1, 2018	$ 0
Increases due to cash received, excluding amounts recognized as revenue during the period	66,619
Decreases due to revenue recognized in the period for which cash has not been received	(1,701)
Balance at December 31, 2018	$ 64,918

Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 125,676	$ 1,195	$ 1,648	$ 641	$ 129,160	$ 0	$ 0
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					5,677
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					123,483
Sanofi Collaboration Agreement \| CNS program license
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					73,932
Sanofi Collaboration Agreement \| Peripheral program license
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					47,148
Sanofi Collaboration Agreement \| Alzheimer's Disease Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					60
Sanofi Collaboration Agreement \| Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue					$ 2,343

License Agreements (Details) - Genentech - USD ($) $ in Millions		12 Months Ended
	Jun. 17, 2016	Dec. 31, 2017	Dec. 31, 2016
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 315.0
License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of maximum reduction as credit for third -party royalty and milestone payments	50.00%
Royalty payment obligations, period	10 years
License Agreement \| Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement, obligation period to use commercially reasonable efforts to research, develop and commercialize at least one licensed product	3 years
Upfront fee paid			$ 8.5
Technology transfer fee			$ 1.5
First clinical milestone		$ 2.5

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 37,172	$ 19,592
Less: accumulated depreciation	(12,010)	(4,669)
Total property and equipment, net	25,162	14,923
Depreciation expense	7,415	3,082	$ 1,469
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,352	11,351
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	22,288	7,737
Computers equipment and purchased software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	458	439
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 74	$ 65

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development expenses	$ 6,643	$ 946
Collaboration accounts receivable	7,364	0
Prepaid insurance	0	927
Accrued interest on short-term marketable securities	1,182	464
Other prepaid and current assets	1,350	1,044
Total prepaid expenses and other current assets	$ 16,539	$ 3,381

Balance Sheet Components - Other Non-Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid Research And Development Expense, Noncurrent	$ 5,841	$ 0
Other investments	0	500
Restricted cash	1,500	451
Other prepaid and non-current assets	764	490
Total other non-current assets	$ 8,105	$ 1,441

Commitments and Contingencies - Narrative (Details) $ in Millions			1 Months Ended	12 Months Ended
	May 02, 2018 USD ($) ft²	Aug. 01, 2016 USD ($) ft²	Aug. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2018 USD ($) ft²	Mar. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Rent expense				$ 6.0	$ 2.2	$ 1.0
Contractual obligation				1.2	1.6
Headquarters Lease
Loss Contingencies [Line Items]
Area under lease \| ft²	148,020	38,109
Lease period	10 years	8 years
Lease renewal option term	10 years
Security deposit	$ 1.5							$ 0.5
New Premisis Sublease Agreement
Loss Contingencies [Line Items]
Rentable square feet \| ft²							36,835
Sublease term							5 years
Total sublease rent amount							$ 14.8
Landlord Funded Tenant Improvements \| Headquarters Lease
Loss Contingencies [Line Items]
Tenant improvement allowance repayable in rent	$ 4.4		$ 1.9	14.6
DMSA
Loss Contingencies [Line Items]
Purchase order executed				24.7	0.7
Costs incurred				3.9	3.9
Payments for development and manufacturing services				3.4	3.4
Non-Cancellable Purchase Commitments
Loss Contingencies [Line Items]
Non-refundable purchase commitments				$ 14.0	$ 0.4
Minimum \| Headquarters Lease
Loss Contingencies [Line Items]
Lease renewal notice period	9 months
Maximum \| Headquarters Lease
Loss Contingencies [Line Items]
Lease renewal notice period	12 months
Maximum \| Landlord Funded Tenant Improvements \| Headquarters Lease
Loss Contingencies [Line Items]
Tenant improvements	$ 25.9	$ 7.4

Commitments and Contingencies - Summary of Company's Future Minimum Lease Commitments (Details) - Headquarters Lease $ in Thousands	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
2019	$ 4,941
2020	9,097
2021	9,716
2022	10,056
2023	10,408
2024 and later	60,882
Total	$ 105,100

Convertible Preferred Stock - Narrative (Details) $ / shares in Units, $ in Millions		1 Months Ended						12 Months Ended
	Dec. 07, 2017 $ / shares	Nov. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jan. 31, 2016 USD ($) shares	Jul. 31, 2015 USD ($) shares	May 31, 2015 USD ($) director shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 USD ($)
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock, par value (usd per share) \| $ / shares								$ 0	$ 0
Common stock, par value (usd per share) \| $ / shares								$ 0	$ 0
Dividends declared and accrued \| $										$ 0.0
Conversion ratio								100.00%
Underwritten IPO stock price (in usd per share) \| $ / shares								$ 5.00
Underwritten IPO gross proceeds \| $								$ 50.0
Convertible preferred stock, shares issued (in shares)								0	0
Convertible preferred stock, shares outstanding (in shares)								0	0
First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock, par value (usd per share) \| $ / shares				$ 0.01
Common stock, par value (usd per share) \| $ / shares				$ 0.01
Common Stock
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock converted into shares of common stock (in shares)									60,365,020
Common Stock \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				81,787,360
Preferred Stock \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				63,288,470
Series A-1
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders \| $ / shares	$ 0.08
Series A-1 \| Initial Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							45,223,970
Number of stock issued (in shares)							6,295,810
Net proceeds \| $							$ 24.8
Series A-1 \| First Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							456,250
Number of stock issued (in shares)							3,481,250
Net proceeds \| $							$ 13.9
Series A-1 \| Second Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)						2,420,830
Net proceeds \| $						$ 9.6
Series A-1 \| Third Additional Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)					500,000
Net proceeds \| $					$ 2.0
Series A-1 \| First Tranche Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)					11,250,000
Net proceeds \| $					$ 45.0
Series A-1 \| Second Tranche Closing and Series A-2 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)				22,166,550
Net proceeds \| $				$ 88.7
Series A-1 \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				46,114,433
Series A-2
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders \| $ / shares	0.16
Series A-2 \| Initial Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)							4,361,530
Series A-2 \| Second Tranche Closing and Series A-2 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)				4,361,530
Net proceeds \| $				$ 34.9
Series A-2 \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				4,361,533
Series B-1
Subsidiary, Sale of Stock [Line Items]
Dividends entitled to be received by preferred stockholders \| $ / shares	0.32
Series B-1 \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				8,125,000
Number of stock issued (in shares)				7,646,240
Closing stock price (usd per share) \| $ / shares				$ 16.00
Net proceeds \| $				$ 122.2
Series B-1 \| Second Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)			478,120
Closing stock price (usd per share) \| $ / shares			$ 16.00
Net proceeds \| $			$ 7.6
Series B-2
Subsidiary, Sale of Stock [Line Items]
Number of stock issued (in shares)		1,764,710
Closing stock price (usd per share) \| $ / shares		$ 17.00
Net proceeds \| $		$ 29.9
Dividends entitled to be received by preferred stockholders \| $ / shares	$ 0.34
Series B-2 \| First Series B-1 Closing
Subsidiary, Sale of Stock [Line Items]
Number of stock authorized (in shares)				4,687,500
Series A
Subsidiary, Sale of Stock [Line Items]
Minimum shares held by investors (in shares)							100,000
Number of members under board election \| director							1
Total number of members \| director							5
Series B
Subsidiary, Sale of Stock [Line Items]
Number of members under board election \| director							1
Series A and Series B
Subsidiary, Sale of Stock [Line Items]
Additional number of members under board election \| director							2

Stock-Based Awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				7 Months Ended	12 Months Ended
	Dec. 31, 2017	Nov. 30, 2015	Aug. 31, 2015	May 31, 2015	Feb. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value of option (usd per share)						$ 14.79	$ 6.41	$ 2.83
Aggregate intrinsic value of options exercised						$ 9,800	$ 3,500	$ 700
Weighted average grant date fair value of options vested (usd per share)						$ 3.85	$ 1.94	$ 1.36
Performance and market contingent stock options granted (in shares)						3,949,778	2,183,365
Exercise price (usd per share)						$ 2.78	$ 1.41
Share based compensation cost recognized						$ 18,791	$ 4,409	$ 2,951
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized						8,698	1,557	873
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized						$ 10,093	2,852	2,078
Performance Contingent Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and market contingent stock options granted (in shares)		500,000	250,000
Exercise price (usd per share)		$ 0.68	$ 0.68
Performance and Market Contingent Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and market contingent stock options granted (in shares)		125,000	1,619,738
Exercise price (usd per share)		$ 0.68	$ 0.68
Vesting trigger, number of consecutive trading days						90 days
Vesting trigger, number of days after IPO						180 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock- based compensation expense related to unvested stock options	$ 300					$ 2,000	$ 300
Expected weighted average period						7 months 6 days	1 year
2017 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)	1,000,000						1,000,000
Common stock purchase discounted rate for employees	85.00%
Maximum employee contribution to ESPP, percent of base compensation	15.00%						15.00%
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock- based compensation expense related to unvested stock options						$ 57,600
Expected weighted average period						3 years
Non-Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized						$ 700	$ 700	$ 1,000
Early Exercised Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reclassification of liability to stockholders' equity						400	100
Unvested early exercised options in non-current liabilities						$ 100	$ 500
2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)	6,379,238						6,379,238
Number of shares available for grant (in shares)	6,012,498					2,289,196	6,012,498
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				8,325,000
Number of shares available for grant (in shares)	0						0	1,813,321
2015 Stock Incentive Plan \| All Shares Granted At $ 0.68
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value of option (usd per share)					$ 1.20
Exercise price (usd per share)					$ 0.68
Shares Transferred As Available For Issuance From 2015 Plan to 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	169,238						169,238
Maximum \| Performance and Market Contingent Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized						$ 6,200
Maximum \| Performance and Market Contingent Stock Options \| General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized						5,800
Maximum \| Performance and Market Contingent Stock Options \| Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized						$ 400
Maximum \| 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
Maximum \| 2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period				10 years
Minimum \| 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date	100.00%
Award vesting period	4 years
Minimum \| 2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date				100.00%
Award vesting period	4 years

Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Options
Number of options, beginning balance (in shares)	6,689,479	5,374,014
Number of options, granted (in shares)	3,949,778	2,183,365
Number of options, exercised (in shares)	(650,471)	(695,192)
Number of options, forfeited (in shares)	(376,134)	(172,708)
Number of options, ending balance (in shares)	9,612,652	6,689,479	5,374,014
Number of options, vested and expected to vest (in shares)	7,867,920
Number of options, exercisable (in shares)	1,933,527
Weighted- Average Exercise Price
Weighted average exercise price, beginning balance (usd per share)	$ 4.08	$ 1.77
Weighted average exercise price, granted (usd per share)	20.25	8.78
Weighted average exercise price, exercised (usd per share)	2.78	1.41
Weighted average exercise price, forfeited (usd per share)	12.48	2.21
Weighted average exercise price, ending balance (usd per share)	10.49	$ 4.08	$ 1.77
Weighted average exercise price, vested and expected to vest (usd per share)	12.66
Weighted average exercise price, exercisable (usd per share)	$ 4.50
Stock Option Activity, Additional Disclosures
Weighted average remaining contractual life	8 years 2 months 12 days	8 years 4 months 13 days	9 years 10 days
Weighted average remaining contractual life, vested and expected to vest	8 years 6 months 14 days
Weighted average remaining contractual life, exercisable	7 years 7 months 20 days
Aggregate intrinsic value, beginning balance	$ 77,317	$ 18,873
Aggregate intrinsic value, ending balance	97,804	$ 77,317	$ 18,873
Aggregate intrinsic value, vested and expected to vest	62,944
Aggregate intrinsic value	$ 31,248

Stock-Based Awards - Summary of Assumptions Used for Estimating the Fair Value of Stock Options Granted (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years 29 days
Volatility, minimum	79.40%	81.90%	91.20%
Volatility, maximum	87.40%	91.30%	92.20%
Risk-free interest rate, minimum	2.60%	1.80%	1.20%
Risk-free interest rate, maximum	3.10%	2.30%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months		6 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days		6 years 29 days

Stock-Based Awards - Summary of Non Employees Assumptions Used for Estimating the Fair Value of Stock Options Granted (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years 29 days
Volatility, minimum	79.40%	81.90%	91.20%
Volatility, maximum	87.40%	91.30%	92.20%
Risk-free interest rate, minimum	2.60%	1.80%	1.20%
Risk-free interest rate, maximum	3.10%	2.30%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	88.90%	86.10%	95.30%
Volatility, maximum	103.10%	88.10%	98.20%
Risk-free interest rate			2.40%
Risk-free interest rate, minimum	2.70%	2.30%
Risk-free interest rate, maximum	3.00%	2.40%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months		6 years
Minimum \| Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	7 years 1 month 20 days	7 years 6 months	8 years 6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days		6 years 29 days
Maximum \| Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	9 years 10 months 9 days	8 years 1 month 2 days	9 years 8 months 12 days

Stock-Based Awards - Summary of Restricted Stock Activity (Details) - Restricted Stock - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Awards & Units
Shares, unvested, beginning balance (in shares)	2,293,788	3,922,638
Shares, granted (in shares)	149,658	0
Shares, vested (in shares)	(1,940,203)	(1,628,850)
Shares, forfeited (in shares)	0	0
Shares, unvested, ending balance (in shares)	503,243	2,293,788
Shares, vested and expected to vest (in shares)	503,243
Weighted-Average Fair Value at Date of Grant per Share
Weighted-average fair value at date of grant per share, unvested, beginning balance (usd per share)	$ 0.18	$ 0.18
Weighted-average fair value at date of grant per share, granted (usd per share)	15.92	0
Weighted-average fair value at date of grant per share, vested (usd per share)	0.18	0.18
Weighted-average fair value at date of grant per share, forfeited (usd per share)	0	0
Weighted-average fair value at date of grant per share, unvested, ending balance (usd per share)	4.86	$ 0.18
Weighted-average fair value at date of grant per share, vested and expected to vest (usd per share)	$ 4.86

Stock-Based Awards - Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted in ESPP (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years 29 days
Volatility, minimum	79.40%	81.90%	91.20%
Volatility, maximum	87.40%	91.30%	92.20%
Risk-free interest rate, minimum	2.60%	1.80%	1.20%
Risk-free interest rate, maximum	3.10%	2.30%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months		6 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days		6 years 29 days
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	61.00%	50.00%
Volatility, maximum	64.30%	51.30%
Risk-free interest rate, minimum	2.10%	1.50%
Risk-free interest rate, maximum	2.70%	1.70%
Dividend yield	0.00%	0.00%
Employee Stock Purchase Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months	5 months 23 days
Employee Stock Purchase Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	1 year	11 months 23 days

Stock-Based Awards - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 18,791	$ 4,409	$ 2,951
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	10,093	2,852	2,078
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 8,698	$ 1,557	$ 873

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Contribution expenses	$ 0.9	$ 0.5

Income Taxes - Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Taxes at the U.S. statutory tax rate	21.00%	34.00%	34.00%
Effect of Tax Act	0.00%	(26.60%)	0.00%
Change in valuation allowance	(20.70%)	(7.30%)	(32.00%)
Contingent consideration issued in tax-free reorganization	0.00%	0.00%	(2.10%)
Research tax credits	10.20%	1.00%	0.60%
Stock-based compensation	(0.70%)	(1.00%)	(0.50%)
Nondeductible acquisition-related costs	(9.50%)	0.00%	0.00%
Other	(0.30%)	(0.10%)	0.00%
Total provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 35,023	$ 37,606
Tax credit carryforwards	9,565	4,072
Reserves and accruals	7,309	3,328
Capitalized start-up costs	4,464	6,248
Intangibles	5,401	5,300
Share based compensation	3,305	733
Other	7	7
Gross deferred tax assets	65,074	57,294
Valuation allowance	(60,915)	(54,650)
Net deferred tax assets	4,159	2,644
Deferred tax liabilities:
Property and equipment	4,159	2,618
Stock-based compensation	0	26
Net deferred tax assets	$ 0	$ 0

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at January 1	$ 1,146	$ 531	$ 122
Additions for tax positions taken in a prior year	0	0	7
Additions for tax positions taken in the current year	1,506	640	411
Reductions for tax positions taken in the prior year	(10)	(25)	(9)
Unrecognized tax benefits at December 31	$ 2,642	$ 1,146	$ 531

Income Taxes - Narrative (Details) - USD ($)	Dec. 22, 2017	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
2017 Tax Act - Deferred tax re-measurement	$ (6,300,000)
Liability recorded for potential interest or penalties		$ 0
Expected change to unrecognized tax benefit		0
Federal authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward		118,600,000
Tax credit carryforward		7,500,000
State authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward		134,000,000.0
Tax credit carryforward		$ 5,700,000

Net Loss and Net Loss Per Share - Summary of Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income (loss)	$ 77,533	$ (35,371)	$ (54,724)	$ (23,678)	$ (22,887)	$ (21,844)	$ (22,134)	$ (21,320)	$ (36,240)	$ (88,185)	$ (86,652)
Denominator:
Weighted average common shares outstanding (in shares)									92,621,991	14,964,144	6,424,720
Net loss per share, basic and diluted (usd per share)					$ (0.74)	$ (2.14)	$ (2.29)	$ (2.36)	$ (0.39)	$ (5.89)	$ (13.49)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss and Net Loss Per Share - Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	10,428,261	9,498,897	68,488,548
Series A-1
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	0	46,114,423
Series A-2
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	0	4,361,527
Series B-1
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	0	8,124,365
Options issued and outstanding and ESPP shares issuable and outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	9,789,594	6,835,313	5,374,014
Restricted shares subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	503,243	2,293,788	3,922,638
Early exercised common stock subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	135,424	369,796	510,417
Shares to be issued under Incro acquisition agreement
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	0	81,164

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 125,676	$ 1,195	$ 1,648	$ 641					$ 129,160	$ 0	$ 0
Profit (loss) from operations	74,722	(37,964)	(57,382)	(25,748)	$ (23,540)	$ (22,288)	$ (22,568)	$ (21,744)	(46,372)	(90,140)	(87,433)
Net income (loss)	$ 77,533	$ (35,371)	$ (54,724)	$ (23,678)	$ (22,887)	$ (21,844)	$ (22,134)	$ (21,320)	$ (36,240)	$ (88,185)	$ (86,652)
Net loss per share, basic (usd per share)	$ 0.82	$ (0.38)	$ (0.59)	$ (0.26)
Net loss per share, diluted (usd per share)	$ 0.79	$ (0.38)	$ (0.59)	$ (0.26)
Net loss per share, basic and diluted (usd per share)					$ (0.74)	$ (2.14)	$ (2.29)	$ (2.36)	$ (0.39)	$ (5.89)	$ (13.49)
Total number of shares outstanding, diluted (in shares)	97,746,224

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details