Maximum employee contribution to ESPP, percent of base compensation Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate Derivative Instruments and Hedging Activities Disclosure [Abstract] Range [Domain] Statistical Measurement [Domain] Percentage of option price of estimated fair value on grant date Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Common stock, par value (usd per share) Common Stock, Par or Stated Value Per Share Net increase (decrease) in cash, cash equivalents and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Significant Accounting Policies Significant Accounting Policies [Text Block] Milestone payments upon achievement of biologic product from each program Aggregate Commercial Milestones Aggregate commercial milestones if one biologic product from each program achieves the milestone. Summary of Available for Sale Securities Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Operating Lease Liabilities, Payments Due [Abstract] Lessee, Operating Lease, Liability, Payment, Due [Abstract] Income Statement Location [Domain] Income Statement Location [Domain] Accounting Policies [Abstract] Unamortized stock- based compensation expense related to unvested stock options Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Cash and Cash Equivalents [Domain] Cash and Cash Equivalents [Domain] Other-than-temporary impairment Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale Purchases of property and equipment Payments to Acquire Property, Plant, and Equipment Operating expenses: Operating Expenses [Abstract] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Share-based Payment Arrangement [Abstract] Concentration of Credit Risk and Other Risks and Uncertainties Concentration Risk, Credit Risk, Policy [Policy Text Block] Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Aggregate intrinsic value, vested and expected to vest Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Additional Paid-in Capital Additional Paid-in Capital [Member] British Pounds Foreign Exchange Forward - British Pounds [Member] Foreign Exchange Forward - British Pounds [Member] Rentable square feet Operating Leases, Sublease, Net Rentable Area Operating Leases, Sublease, Net Rentable Area Total liability fair value measurements Financial and Nonfinancial Liabilities, Fair Value Disclosure Common stock, shares outstanding (in shares) Common stock, shares outstanding beginning balance (in shares) Common stock, shares outstanding ending balance (in shares) Common Stock, Shares, Outstanding Cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Entity Ex Transition Period Entity Ex Transition Period Income Taxes Income Tax, Policy [Policy Text Block] Weighted- Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Risk-free interest rate, maximum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Operating activities Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Vesting trigger, number of days after IPO Share-Based Compensation Arrangement By Share-Based Payment Award, Award Vesting, Days After Initial Public Offering Share-Based Compensation Arrangement By Share-Based Payment Award, Award Vesting, Days After Initial Public Offering Aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Revenue recognized Contract with Customer, Liability, Revenue Recognized Net gain (loss) on derivative instruments Derivative, Gain (Loss) on Derivative, Net Landlord Funded Tenant Improvements Landlord Funded Tenant Improvements [Member] Landlord funded tenant improvements. Property and equipment, net Property, Plant and Equipment, Net Aggregate intrinsic value, beginning balance Aggregate intrinsic value, ending balance Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Level 2 Fair Value, Inputs, Level 2 [Member] Convertible preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Weighted-average fair value at date of grant per share, vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Collaborative Arrangement Collaborative Arrangement [Member] Weighted-average fair value at date of grant per share, vested and expected to vest (usd per share) Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value Expected To Vest Share based compensation arrangement by share based payment award equity instruments other than options nonvested weighted average grant date fair value expected to vest. Weighted average exercise price, vested and expected to vest (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price Options Issued and Outstanding and ESPP Shares Issuable and Outstanding Share-based Payment Arrangement, Option [Member] Restricted cash included within prepaid expenses and other current assets Restricted Cash and Cash Equivalents, Current Number of options, vested and expected to vest (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number Expected term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Number of options, forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Statement [Table] Statement [Table] Number of Contracts Derivative, Number of Instruments Held Fair market value of common stock Common Stocks, Including Additional Paid in Capital Aggregated option exercise fee Aggregate Option Exercise Fee Aggregate option exercise fee. Commercial paper Commercial Paper, Not Included with Cash and Cash Equivalents [Member] Investing activities Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Number of shares available for grant (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Preferred Stock Preferred Stock [Member] Operating lease costs Lease, Cost Accumulated deficit Retained Earnings (Accumulated Deficit) Weighted-average discount rate (as a percentage) Operating Lease, Weighted Average Discount Rate, Percent Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Volatility, maximum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Maximum Other comprehensive income (loss): Other Comprehensive Income (Loss), Tax [Abstract] Document Quarterly Report Document Quarterly Report Cash and Cash Equivalents [Abstract] Takeda Collaboration Agreement Takeda Collaboration Agreement [Member] Takeda Collaboration Agreement Long-term marketable securities: Long Term Marketable Securities [Member] Long term marketable securities. Restricted cash included within other non-current assets Restricted Cash and Cash Equivalents, Noncurrent Preclinical milestone payment received Preclinical Milestone Payment Received Preclinical milestone payment received. Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Supplemental disclosures of cash flow information Supplemental Cash Flow Information [Abstract] 2022 Lessor, Operating Lease, Payments to be Received, Four Years 2022 Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Four Share based compensation cost recognized Total Share-based Payment Arrangement, Expense Financial Instruments [Domain] Financial Instruments [Domain] Sublease term Lessor, Operating Lease, Term of Contract Operating lease liability, less current portion Operating Lease, Liability, Noncurrent Assets Assets [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Aggregate Notional Amount in Foreign Currency Derivative, Notional Amount Leasehold improvements Leasehold Improvements, Gross Long-term marketable securities Marketable Securities, Noncurrent Range [Axis] Statistical Measurement [Axis] Derivative asset Derivative Asset Convertible preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Accounts payable Increase (Decrease) in Accounts Payable New Premises Sublease Agreement New Premises Sublease Agreement [Member] New Premises Sublease Agreement [Member] Contingent consideration recognized Asset Acquisition, Contingent Consideration, Amount Recognized Asset Acquisition, Contingent Consideration, Amount Recognized Document Fiscal Year Focus Document Fiscal Year Focus Stockholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Option fee Option Fees Per Program Option fees per program. Effective maturity (less than) Debt Securities, Available-for-sale, Term Fair Value Measurements Fair Value Disclosures [Text Block] Takeda Pharmaceutical Company Limited Takeda Pharmaceutical Company Limited [Member] Takeda pharmaceutical company limited. Other non-current assets Other Assets, Noncurrent Property Subject to or Available for Operating Lease [Axis] Property Subject to or Available for Operating Lease [Axis] Entity Current Reporting Status Entity Current Reporting Status Convertible preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Deferred rent, less current portion Deferred Rent Credit, Noncurrent Entity Emerging Growth Company Entity Emerging Growth Company Accumulated Deficit Retained Earnings [Member] Tenant improvements provided by the landlord Noncash Tenant Improvements Paid By Landlord Noncash Tenant Improvements Paid By Landlord Net cash provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Number of performance obligations Revenue, Number Of Performance Obligations Revenue, Number Of Performance Obligations Schedule of Restricted Cash and Cash Equivalents Restrictions on Cash and Cash Equivalents [Table Text Block] Common stock purchase discounted rate for employees Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Offering Date Proceeds from clinical milestone Proceeds For Clinical Milestone Proceeds For Clinical Milestone Premium on sale of common stock Sale Of Stock Consideration Premium Received On Transaction Sale of stock consideration premium received on transaction. Anti-dilutive securities (shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Purchase order executed Purchase Order Executed Purchase order executed. Early exercised common stock subject to future vesting Early exercised common stock subject to future vesting [Member] Early exercised common stock subject to future vesting. Lease renewal option term Lessee, Operating Lease, Renewal Term 2020 Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Two Total undiscounted lease payments Lessee, Operating Lease, Liability, Payments, Due Restricted Stock Activity Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Number of common stock (in shares) Sale of Stock, Number of Shares Issued in Transaction Vesting of early exercised common stock Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Risk-free interest rate, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum Short-term marketable securities Marketable Securities, Current Number of indications Number Of Indications Number Of Indications Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Entity Address, State or Province Entity Address, State or Province Summary of Stock Option Activity Share-based Payment Arrangement, Option, Activity [Table Text Block] Derivative Financial Instruments Derivative Instruments and Hedging Activities Disclosure [Text Block] Purchase Commitment, Excluding Long-term Commitment [Domain] Purchase Commitment, Excluding Long-term Commitment [Domain] Variable consideration relating to future milestones Variable Consideration Future Consideration Variable consideration relating to future milestones. Increase in transaction price Collaboration Agreement, Transaction Price, Increase Collaboration Agreement, Transaction Price, Increase Recently Issued and Recently Adopted Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Document Transition Report Document Transition Report Sublease income Operating Leases, Rent Expense, Sublease Rentals Shares, vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Financing activities Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Short-term marketable securities: Short-term Investments [Member] Total operating expenses Operating Expenses Deferred revenue Deferred Revenue Weighted average exercise price, granted (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Cash, cash equivalents and restricted cash at beginning of period Cash, cash equivalents and restricted cash at end of period Total cash, cash equivalents, and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Alzheimer's Disease Services Alzheimer's Disease Services [Member] Alzheimer's Disease Services Estimated fair value of option (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Swiss Francs Foreign Exchange Forward - Swiss Francs [Member] Foreign Exchange Forward - Swiss Francs [Member] Share Awards & Units Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] Weighted average remaining contractual life Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Stock–based compensation expense Share-based Payment Arrangement, Noncash Expense Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Award expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Arrangements and Non-arrangement Transactions [Domain] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Area under lease Operating Leases Area Under Lease Operating leases area under lease. Cash paid for lease liabilities Operating Lease, Payments Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Stock issued during period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period Weighted-Average Fair Value at Date of Grant per Share Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Level 3 Fair Value, Inputs, Level 3 [Member] Entity Address, City or Town Entity Address, City or Town Derivatives and Hedging Activities Derivatives, Policy [Policy Text Block] Purchases of marketable securities Payments to Acquire Marketable Securities Entity Filer Category Entity Filer Category Total aggregate payments due upon achievement of certain preclinical milestone events Aggregate Preclinical Milestone Payment Total aggregate payments due upon achievement of certain preclinical milestone events. Designated as Hedging Instrument Designated as Hedging Instrument [Member] Share based compensation cost not recognized Performance And Market Share Based Compensation Compensation Cost Not Yet Recognized Performance and market share based compensation cost not yet recognized. Commercial paper Commercial Paper [Member] Derivative [Table] Derivative [Table] Loss before income taxes Income (Loss) Attributable to Parent, before Tax Type of Arrangement and Non-arrangement Transactions [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Statement [Line Items] Statement [Line Items] Milestone payments per biologic product upon achievement of a certain sales-based milestone Commercial Milestones Per Program Commercial milestones per program. Target option period Target Option Period Target Option Period Entity Registrant Name Entity Registrant Name Restricted shares subject to future vesting Restricted shares subject to future vesting [Member] Restricted shares subject to future vesting. Preclinical milestone payment earned not yet received Preclinical Milestone Payment Earned Not Yet Received Preclinical Milestone Payment Earned Not Yet Received Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Amendment Flag Amendment Flag Upfront payment Upfront Payments Upfront payments. City Area Code City Area Code Issuance of common stock in connection with the Takeda Collaboration Agreement Stock Issued During Period, Value, Collaboration Agreement Stock Issued During Period, Value, Collaboration Agreement Common stock reserved for issuance (in shares) Common Stock, Capital Shares Reserved for Future Issuance Entity Central Index Key Entity Central Index Key Segments Segment Reporting, Policy [Policy Text Block] Payments of issuance costs related to issuance of stock Payments of Stock Issuance Costs Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Accounting Standards Update 2016-02 Accounting Standards Update 2016-02 [Member] Weighted average number of shares outstanding, basic and diluted (in shares) Weighted Average Number of Shares Outstanding, Basic and Diluted Gain on disposal of property and equipment Gain (Loss) on Disposition of Property Plant Equipment Number of Options Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Common stock, shares authorized (in shares) Common Stock, Shares Authorized Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Weighted average remaining contractual life, exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term Discount rate (as a percentage) Lessee, Operating Lease, Discount Rate Product and Service [Domain] Product and Service [Domain] Milestone payments upon achievement of specified clinical and regulatory milestones License Agreement Milestone Payment Upon Achievement Of Clinical And Regulatory Milestones License agreement milestone payment upon achievement of clinical and regulatory milestones. Type of Adoption [Domain] Type of Adoption [Domain] Headquarters Lease Headquarters Lease [Member] Headquarters lease. Issuances under equity incentive plans Stock Issued During Period, Value, Equity Incentive Plan Stock Issued During Period, Value, Equity Incentive Plan Fair Value Disclosures [Abstract] Entity Shell Company Entity Shell Company Volatility, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Minimum Clinical milestone payment Clinical Milestone Payment Clinical Milestone Payment Acquisition Asset Acquisition [Text Block] Asset Acquisition [Text Block] Total stockholders' equity Beginning balance Ending balance Stockholders' Equity Attributable to Parent Rent expense Operating Leases, Rent Expense, Net Initial option exercise payments Initial option exercise payments Initial option exercise payments. Net unrealized income (loss) on marketable securities, net of tax Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Reclassification Adjustments, after Tax Denali Denali [Member] Denali [Member] Plan Name [Axis] Plan Name [Axis] Investments [Domain] Investments [Domain] Equity Component [Domain] Equity Component [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Common Stock Common Stock [Member] Weighted average exercise price, exercisable (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price Lease Arrangement, Type [Axis] Lease Arrangement, Type [Axis] License Agreement License Agreement [Member] License agreement. Registration Payment Arrangement, Arrangement [Domain] Registration Payment Arrangement, Arrangement [Domain] Derivative [Line Items] Derivative [Line Items] Foreign Exchange Contracts Foreign Exchange Contract [Member] 2017 Employee Stock Purchase Plan Two Thousand And Seventeen Employee Stock Purchase Plan [Member] 2017 employee stock purchase plan. Number of options, exercisable (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number 2021 Lessor, Operating Lease, Payments to be Received, Three Years Accounts payable Accounts Payable, Current Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Current Fiscal Year End Date Current Fiscal Year End Date Hedging Designation [Axis] Hedging Designation [Axis] Aggregate intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Restricted Stock Restricted Stock [Member] Summary of Forward Foreign Currency Exchange Purchase Contracts Outstanding Schedule of Derivative Instruments [Table Text Block] Equity Award [Domain] Award Type [Domain] Purchase Commitment, Excluding Long-term Commitment [Axis] Purchase Commitment, Excluding Long-term Commitment [Axis] Stock Option Activity, Additional Disclosures Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Weighted-average fair value at date of grant per share, granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Purchase price Sale of Stock, Consideration Received on Transaction Non-cash rent expense Increase (Decrease) In Non-Cash Operating Lease Expense Increase (Decrease) In Non-Cash Operating Lease Expense Loss Contingencies [Table] Loss Contingencies [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Number of options, granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Level 1 Fair Value, Inputs, Level 1 [Member] Amortized Cost Debt Securities, Available-for-sale, Amortized Cost Proceeds from exercise of awards under equity incentive plans Proceeds, Issuance of Shares, Share-based Payment Arrangement, Including Option Exercised Issuances under equity incentive plans (in shares) Stock Issued During Period, Shares, Equity Incentive Plan Stock Issued During Period, Shares, Equity Incentive Plan Stock-Based Awards Share-based Payment Arrangement [Text Block] General and Administrative General and Administrative Expense [Member] Closing stock price (usd per share) Sale of Stock, Price Per Share Milestones recognized Revenue Recognition, Milestone Method, Revenue Recognized Income Statement Location [Axis] Income Statement Location [Axis] 2017 Equity Incentive Plan Two Thousand Seventeen Equity Incentive Plan [Member] Two thousand seventeen equity incentive plan. Shares, forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Performance and market-contingent awards expense Performance And Market Share Based Compensation Performance And Market Share Based Compensation Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Vesting trigger, number of consecutive trading days Share-Based Compensation Arrangement By Share-Based Payment Award, Award, Threshold Consecutive Trading Days Share-Based Compensation Arrangement By Share-Based Payment Award, Award, Threshold Consecutive Trading Days Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities Number of options, beginning balance (in shares) Number of options, ending balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Investment Type [Axis] Investment Type [Axis] 2015 Stock Incentive Plan Two Thousand Fifteen Equity Incentive Plan [Member] Two thousand fifteen equity incentive plan. Salary and Wage, Excluding Cost of Good and Service Sold [Abstract] Significant Accounting Policies [Line Items] Significant Accounting Policies [Line Items] Significant accounting policies. Loss from operations Operating Income (Loss) Net operating lease liabilities Operating Lease, Liability Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Commitments and Contingencies Disclosure [Abstract] Expected weighted average period Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Net loss per share, basic and diluted (usd per share) Earnings Per Share, Basic and Diluted Loss Contingencies [Line Items] Loss Contingencies [Line Items] Costs incurred Costs Incurred, Development Costs Shares, unvested, beginning balance (in shares) Shares, unvested, ending balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Shares, vested and expected to vest (in shares) Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested And Expected To Vest Outstanding Number Share based compensation arrangement by share based payment award equity instruments other than options vested and expected to vest outstanding number. Weighted average remaining contractual life, vested and expected to vest Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term Accrued compensation Employee-related Liabilities, Current Total asset fair value measurements Assets, Fair Value Disclosure Issuance of common stock in connection with Takeda Collaboration Agreement (in shares) Stock Issued During Period, Shares, Collaboration Agreement Stock Issued During Period, Shares, Collaboration Agreement 2024 and later Lessor, Operating Lease, Payments to be Received, Thereafter Commitments and contingencies Commitments and Contingencies Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Cover page. 2019 (three months) Lessor, Operating Lease, Payments to be Received, Remainder of Fiscal Year Non-cash rent expense Increase Decrease In Deferred Rent Amount of increase (decrease) of deferred rent. Research and development Research and Development Expense Period of recognition Performance And Market Share Based Compensation Compensation Cost Not Yet Recognized Period Of Recognition Performance And Market Share Based Compensation Compensation Cost Not Yet Recognized Period Of Recognition Total liabilities Liabilities Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Present value adjustment Lessee, Operating Lease, Liability, Undiscounted Excess Amount Other non-cash items Other Noncash Income (Expense) U.S. government treasuries US Treasury Securities [Member] Trading Symbol Trading Symbol Property and equipment purchases accrued but not yet paid Capital Expenditures Incurred but Not yet Paid Document Period End Date Document Period End Date Receivable Accounts Receivable, after Allowance for Credit Loss Contract liability Contract with Customer, Liability Project [Axis] Project [Axis] Common stock, $0.01 par value;400,000,000 shares authorized as of September 30, 2019 and December 31, 2018; 95,987,607 shares and 94,662,435 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively Common Stock, Value, Issued Research and Development Research and Development Expense [Member] Sanofi Collaboration Agreement Sanofi Collaboration Agreement [Member] Sanofi Collaboration Agreement Schedule of Cash and Cash Equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Significant Accounting Policies [Table] Significant Accounting Policies [Table] Significant Accounting Policies [Table] Dividends paid Dividends Maximum development plan period for research costs Maximum development plan period for research costs Maximum development plan period for research costs. Number of additional shares allowable under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Allowable Under The Plan Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Allowable Under The Plan Milestone Triggered Milestone Triggered [Member] Milestone Triggered Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Preclinical milestone payments per program Preclinical Milestone Payment Per Program Payments due upon achievement of certain preclinical milestone events per program. Restricted Cash Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Shares Transferred As Available For Issuance From 2015 Plan to 2017 Plan Shares Transferred As Available For Issuance From Two Thousand Fifteen Plan to Two Thousand Seventeen Plan [Member] Shares transferred as available for issuance from the 2015 plan to the 2017 plan. Operating Lease Maturity, Receivable Lessor, Operating Lease, Payments to be Received, Maturity [Table Text Block] Stock-based compensation APIC, Share-based Payment Arrangement, Increase for Cost Recognition Total assets Assets Marketable securities Debt Securities, Available-for-sale Cash and Cash Equivalents [Axis] Cash and Cash Equivalents [Axis] 2023 Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Five Project [Domain] Project [Domain] Plan Name [Domain] Plan Name [Domain] Genentech Inc Genentech Inc [Member] Genentech Inc. Vesting of restricted stock awards and units Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures Performance obligation Revenue, Remaining Performance Obligation, Amount 2024 and later Lessee, Operating Lease, Liability, Payments, Due after Rolling Year Five Document Fiscal Period Focus Document Fiscal Period Focus Document Type Document Type Assets Measured at Fair Value Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Tax Identification Number Entity Tax Identification Number Income Statement [Abstract] Product Product [Member] Entity Interactive Data Current Entity Interactive Data Current Number of financing leases Lessee, Finance Lease, Number Of Contracts Lessee, Finance Lease, Number Of Contracts Accrued and other current liabilities Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Corporate debt securities Corporate Debt Securities [Member] Number of operating segments Number of Operating Segments Use of Estimates Use of Estimates, Policy [Policy Text Block] Income tax benefit Income Tax Expense (Benefit) Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Total current assets Assets, Current Operating Lease Maturity, Payments Lessee, Operating Lease, Liability, Maturity [Table Text Block] Hedging Designation [Domain] Hedging Designation [Domain] Contract liabilities, less current portion Contract with Customer, Liability, Noncurrent Upfront option fee Upfront Option Fee Upfront option fee. Total current liabilities Liabilities, Current General and administrative General and Administrative Expense License Agreements License Agreement [Text Block] License Agreement Non-refundable purchase commitments Purchase Obligation Total aggregate payments due upon achievement of certain clinical and regulatory milestone events Aggregate Development Milestone Payment Aggregate development milestone payment. Local Phone Number Local Phone Number Total liabilities and stockholders’ equity Liabilities and Equity Additional paid-in capital Additional Paid in Capital Entity Common Stock, Shares Outstanding (shares) Entity Common Stock, Shares Outstanding Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Sanofi Sanofi [Member] Sanofi [Member] Equity Components [Axis] Equity Components [Axis] Cash and Marketable Securities Cash, Cash Equivalents, and Marketable Securities [Text Block] Shares, granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Weighted-average fair value at date of grant per share, forfeited (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Summary of Assumptions Used for Estimating the Fair Value of Stock Granted Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Award Type [Axis] Award Type [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Property Subject to or Available for Operating Lease [Domain] Property Subject to or Available for Operating Lease [Domain] Payments for development and manufacturing services Payments for Manufacturing Cost Payments for manufacturing cost. Exercise of buy-out option Exercise of buy-out option [Member] Exercise of buy-out option. Retained Activities Retained Activities [Member] Retained Activities Liability - foreign currency derivative contracts Derivative liability Derivative Liability Registration Payment Arrangement by Arrangement [Axis] Registration Payment Arrangement by Arrangement [Axis] Maturities of marketable securities Proceeds from Sale and Maturity of Marketable Securities Short-term marketable securities: Short Term Marketable Securities [Member] Short term marketable securities. Common stock, shares issued (in shares) Common Stock, Shares, Issued Operating lease right-of-use asset Operating Lease, Right-of-Use Asset Convertible preferred stock, par value (usd per share) Preferred Stock, Par or Stated Value Per Share Weighted average exercise price, beginning balance (usd per share) Weighted average exercise price, ending balance (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Convertible preferred stock, $0.01 par value; 40,000,000 shares authorized as of September 30, 2019 and December 31, 2018; 0 shares issued and outstanding as of September 30, 2019 and December 31, 2018 Preferred Stock, Value, Issued Amount paid Collaborative Arrangement, Payment Collaborative Arrangement, Payment Net loss Net loss Net Income (Loss) Attributable to Parent Remaining preclinical milestones, cost and profit sharing income, and the development and commercial milestones Constrained Variable Consideration Constrained variable consideration. Adjustments to reconcile net loss to net cash used in operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Entity Small Business Entity Small Business Entity File Number Entity File Number Contingent consideration payment Contingent Consideration Payment Contingent Consideration Payment Leases Lessee, Leases [Policy Text Block] Funded percentage Collaborative Agreement, Funded Percentage Collaborative Agreement, Funded Percentage Operating lease liability, current Operating Lease, Liability, Current Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Principles of Consolidation Consolidation, Policy [Policy Text Block] Performance and Market Contingent Stock Options Performance And Market Contingent Stock Options [Member] Performance and market contingent stock options. Weighted average exercise price, forfeited (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Accrued liabilities Accrued Liabilities, Current Research and development expense Research and Development in Process Options issued and outstanding and ESPP shares issuable Employee And Non Employee Stock Option [Member] Employee And Non Employee Stock Option [Member] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Summary of Stock-Based Compensation Expense Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Counterparty Name [Axis] Counterparty Name [Axis] Award vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Regulatory milestone payment Regulatory Milestone Payment Regulatory Milestone Payment Product and Service [Axis] Product and Service [Axis] Financial Instrument [Axis] Financial Instrument [Axis] Percent of outstanding shares Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum Depreciation and amortization Depreciation, Depletion and Amortization Number of targets Number Of Targets Number Of Targets Tenant improvement allowance repayable in rent Tenant Improvement Allowance Repayable As Rent Portion of tenant improvement allowance repayable to landlord in future rent. Schedule of Collaborative Arrangements and Non-collaborative Arrangement Transactions [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Lease Arrangement, Type [Domain] Lease Arrangement, Type [Domain] Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member] Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member] Interest and other income, net Nonoperating Income (Expense) Contract liabilities Increase (Decrease) in Contract with Customer, Liability U.S. government agency securities US Government Agencies Debt Securities [Member] Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Research and Development Arrangement, Contract to Perform for Others, Type [Axis] Research and Development Arrangement, Contract to Perform for Others, Type [Axis] Current liabilities: Liabilities, Current [Abstract] Lease renewal notice period Lessee, Operating Lease, Renewal Notice Period Lessee, Operating Lease, Renewal Notice Period Unrealized Holding Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Total undiscounted sublease receipts Lessor, Operating Lease, Payments to be Received DMSA DMSA [Member] DMSA. Number of options, exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Weighted average grant date fair value of options vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value Liabilities and stockholders' equity Liabilities and Equity [Abstract] Weighted-average fair value at date of grant per share, unvested, beginning balance (usd per share) Weighted-average fair value at date of grant per share, unvested, ending balance (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Collaboration revenue Sales Revenue from Contract with Customer, Excluding Assessed Tax CNS Product CNS Product [Member] CNS Product Contingent payments upon the achievement of milestones Contingent payments upon achievement of milestones Contingent payments upon the achievement of milestones. Money market funds Money Market Funds [Member] Upfront fee paid Upfront Fees Paid Upfront fees paid. Commercial milestone payments Commercial Milestone Payments Commercial Milestone Payments Milestone payments upon achievement of certain clinical, regulatory and sales milestone events Aggregate Clinical Regulatory And Sales Milestone Payment Aggregate Clinical Regulatory And Sales Milestone Payment Lease period Lessee, Operating Lease, Term of Contract 2019 (three months) Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year Share Purchase Agreement Share Purchase Agreement [Member] Share purchase agreement. Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Unrealized Holding Losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Net Loss per Share Earnings Per Share, Policy [Policy Text Block] Derivative Contract [Domain] Derivative Contract [Domain] Contract liabilities Contract with Customer, Liability, Current Earnings Per Share [Abstract] Revenue Recognition Revenue [Policy Text Block] Exercise price (usd per share) Weighted average exercise price, exercised (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-sale [Table] Minimum Minimum [Member] Long-term marketable securities: Long Term Investments [Member] Long term investments. Security Exchange Name Security Exchange Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code 2021 Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Three Entity Address, Address Line One Entity Address, Address Line One Peripheral Product Peripheral Program License Peripheral Product [Member] Peripheral Product Current assets: Assets, Current [Abstract] Summary of Collaboration Revenue Disaggregation of Revenue [Table Text Block] Vesting of early exercised common stock (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture 2023 Lessor, Operating Lease, Payments to be Received, Five Years Issuance of common stock in connection with the Takeda Collaboration Agreement Proceeds From Issuance Of Common Stock In Connection With Collaboration Agreement Proceeds From Issuance Of Common Stock In Connection With Collaboration Agreement Other non-current liabilities Other Liabilities, Noncurrent Net Loss Per Share Earnings Per Share [Text Block] Collaboration Agreements Collaborative Arrangement Disclosure [Text Block] Euros Foreign Exchange Forward - Euros [Member] Foreign Exchange Forward - Euros [Member] Number of programs Number Of Programs Number Of Programs Vesting of restricted stock awards and units (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures Net amortization of discounts on marketable securities Investment Income, Net, Amortization of Discount and Premium Other current liabilities Other Liabilities, Current Organization and Description of Business Organization and Description of Business, Policy [Policy Text Block] Organization and description of business, policy. 2020 Lessor, Operating Lease, Payments to be Received, Two Years Maximum Maximum [Member] Title of 12(b) Security Title of 12(b) Security