Cash and Cash Equivalents Cash and Cash Equivalents, Unrestricted Cash and Cash Equivalents, Policy [Policy Text Block] Customer Customer [Domain] Letter of Credit Letter of Credit [Member] Net Loss per Share Earnings Per Share, Policy [Policy Text Block] Leasehold improvements Leasehold Improvements [Member] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Net loss Net loss Net loss Net Income (Loss) Attributable to Parent Entity Emerging Growth Company Entity Emerging Growth Company Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Accrued expenses Accrued Expenses Current Accrued expenses current. Income Statement Location [Axis] Income Statement Location [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Entity Address, Address Line One Entity Address, Address Line One Related party contract asset Increase (Decrease) in Contract with Customer, Asset Segment and Geographical Information Segment Reporting, Policy [Policy Text Block] Additional paid-in capital Additional Paid in Capital, Common Stock Entity Filer Category Entity Filer Category Foreign Currency Transactions Foreign Currency Transactions and Translations Policy [Policy Text Block] Property and equipment purchases accrued and not yet paid Property and Equipment Purchased, Accrued But Not Yet Paid Property and Equipment Purchased, Accrued But Not Yet Paid Issuance of common stock shares, under Open Market Agreement, net of issuance cost (in shares) Stock Issued During Period Shares Issuance Of Under Open Market Agreement Stock Issued During Period Shares Issuance Of Under Open Market Agreement Options grants (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Excluding Shares Issued in Connection with Acquisition Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Excluding Shares Issued in Connection with Acquisition Other non-current assets Other Assets, Noncurrent Net (decrease) increase in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Stock-based compensation expense APIC, Share-based Payment Arrangement, Increase for Cost Recognition Intrinsic value of stock options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Document Fiscal Year Focus Document Fiscal Year Focus Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Amount of fund received for research activities Amount Of Fund Received For Research Activities Amount of fund received for research activities. Funded percentage of global development costs Research And Development Collaboration Agreement, Global Development Costs, Funded Percentage Research And Development Collaboration Agreement, Global Development Costs, Funded Percentage Options to purchase common stock Options To Purchase Common Stock [Member] Options to purchase common stock. Amount of additional fund received for research activities Amount Of Additional Fund Received For Research Activities Amount of additional fund received for research activities Potentially dilutive securities not included in calculations of diluted per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Research and development Research and Development Expense [Member] Sale of Stock [Axis] Sale of Stock [Axis] Sale of Stock [Axis] Options cancelled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Cancelled in Period Share-based Compensation Arrangement by Share-based Payment Award, Options, Cancelled in Period Restricted Cash Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Title of Individual [Domain] Title of Individual [Domain] Aggregate Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding Award Type [Domain] Award Type [Domain] Summary Of Significant Accounting Policies [Line Items] Summary Of Significant Accounting Policies [Line Items] Summary of significant accounting policies. Credit Facility [Domain] Credit Facility [Domain] Assets Assets [Abstract] Summary of Cash and Cash Equivalents and Marketable Securities Classified as Available-for-sale Securities Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Common stock options outstanding Common Stock Options Outstanding [Member] Common stock options outstanding. Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Exercisable (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Merck Sharp & Dohme Corp Merck Sharp And Dohme Corporation [Member] Merck Sharp and Dohme Corporation. Percentage of revenue Concentration Risk, Percentage Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding as of September 30, 2021 and December 31, 2020, respectively Preferred Stock, Value, Issued Property and Equipment, Net Property, Plant and Equipment, Policy [Policy Text Block] Beginning balance (in shares) Ending balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Entity Address, City or Town Entity Address, City or Town Accumulated Deficit Retained Earnings [Member] Prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Area of leased property Area of Real Estate Property Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Estimated Useful Life of Asset Schedule Of Estimated Useful Life Of Asset Table [Table Text Block] Schedule of estimated useful life of asset. Maximum percentage of payroll deductions of employee's compensation Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate Manufacturing costs Accrued Manufacturing Costs Current Accrued Manufacturing costs current. Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Entity Interactive Data Current Entity Interactive Data Current Number of operating business segment Number of Operating Segments Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Net loss per share, basic (in dollars per share) Earnings Per Share, Basic Proceeds from employee stock purchase plan Proceeds, Issuance of Shares, Share-based Payment Arrangement, Excluding Option Exercised Laboratory equipment and office furniture Laboratory Equipment And Office Furniture [Member] Laboratory equipment and office furniture. Document Type Document Type Entity Current Reporting Status Entity Current Reporting Status Commercial paper Commercial Paper [Member] Other non-cash expenses Other Noncash Income (Expense) Research and Development Research and Development Expense, Policy [Policy Text Block] Issuance of common stock to participants in 401(k) plan Stock Issued During Period, Value, Employee Benefit Plan Assets: Assets, Fair Value Disclosure [Abstract] Fair value of financial assets Assets, Fair Value Disclosure Level 1 Fair Value, Inputs, Level 1 [Member] Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Document Transition Report Document Transition Report Denominator: Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract] Merck Merck [Member] Merck. Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Cash and cash equivalents Cash and Cash Equivalents [Member] Total liabilities and stockholders' equity Liabilities and Equity Contract liabilities Contract with Customer, Liability, Current Scenario [Axis] Scenario [Axis] Options exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Vested and expected to vest, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Intrinsic Value U.S. government agencies securities US Government Agencies Debt Securities [Member] Liabilities and stockholders' equity Liabilities and Equity [Abstract] Accounts payable Increase (Decrease) in Accounts Payable Common stock options available for grant Common Stock Options Available For Grant [Member] Common stock options available for grant. Operating expenses: Operating Expenses [Abstract] Issuance of common stock upon exercise of stock options Stock Issued During Period, Value, Stock Options Exercised Minimum Minimum Minimum [Member] Collaborative arrangement funding, amount Collaborative Arrangement Annual Funding Amount Over Agreement Term Collaborative arrangement annual funding amount over agreement term. Organization and Description of Business Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Number of options to extend performance of research program Number Of Options To Extend Performance Of Research Program Number of options to extend performance of research program. 2023 Operating Leases, Future Minimum Payments, Due in Two Years Schedule of Shares of Common Stock Reserved for Issuance Schedule of Stock by Class [Table Text Block] Other comprehensive loss, net of tax: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Gross Unrealized Gain Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Summary of significant accounting policies Trading Symbol Trading Symbol ESPP shares available for purchase Four Zero One K Matching Plan [Member] Four zero one k matching plan. Current liabilities: Liabilities, Current [Abstract] Issuance of common stock under offering, net of issuance costs Stock Issued During Period, Value, New Issues General and administrative General and Administrative Expense Beginning balance (in shares) Ending balance (in shares) Shares, Outstanding Exercisable, weighted average exercise price (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Percentage of purchase of common stock fair market value Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Offering Date Level 3 Fair Value, Inputs, Level 3 [Member] Leases Lessee, Leases [Policy Text Block] Capitalized Contract Cost [Domain] Capitalized Contract Cost [Domain] Accounting Policies [Abstract] Accounting Policies [Abstract] Weighted Average Remaining Contractual Life (In Years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Deferred offering costs paid Payments of Stock Issuance Costs Issuance of common stock under offering, net of issuance cost (in shares) Number of shares issued in transaction (in shares) Stock Issued During Period, Shares, New Issues Tenant improvement allowances Tenant Improvements Common stock, $0.001 par value; 400,000,000 shares authorized; 77,642,628 and 70,585,364 shares issued and outstanding as of September 30, 2021 and December 31, 2020, respectively Common Stock, Value, Issued Customer Concentration Risk Customer Concentration Risk [Member] Basis of Presentation and Principles of Consolidation Basis of Accounting, Policy [Policy Text Block] Commitments and contingencies (Note 6) Commitments and Contingencies Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income (Loss), Net of Tax Options cancelled (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Cancelled in Period, Weighted Average Exercise Price Share-based Compensation Arrangements by Share-based Payment Award, Options, Cancelled in Period, Weighted Average Exercise Price Aggregate offering price Sale of Stock, Aggregate Offering Price Sale of Stock, Aggregate Offering Price Clinical Development Event [Axis] Clinical Development Event [Axis] Clinical Development Event Antidilutive Securities, Name Antidilutive Securities, Name [Domain] Document Period End Date Document Period End Date Related party receivable from collaboration Due from Related Parties, Current Clinical trials and research and development costs Accrued Clinical Trials And Research And Development Costs Current Accrued clinical trials and research and development costs current. Marketable Securities Marketable Securities, Policy [Policy Text Block] Entity Registrant Name Entity Registrant Name Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] R&D tail period Research And Development Service Tail Period Research And Development Service Tail Period Aggregate net proceeds Sale of Stock, Consideration Received on Transaction Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Related party revenue Related party revenue Revenue from Related Parties Financial Instrument [Axis] Financial Instrument [Axis] Deferred rent, non-current Deferred Rent Credit, Noncurrent Number of purchase periods Number Of Purchase Periods Number of purchase periods. Accrued liabilities Accrued Liabilities, Current Related party revenue Collaboration and license revenue Revenue from Contract with Customer, Excluding Assessed Tax Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] First Indication First Indication [Member] First indication. Equity [Abstract] Equity [Abstract] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Comprehensive Loss Comprehensive Income, Policy [Policy Text Block] Vesting of common stock from early exercises Vesting Of Common Stock From Early Exercises Vesting of common stock from early exercises. Common Stock Common Stock [Member] Loss from operations Operating Income (Loss) Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Gross Unrealized Loss Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Stock options, early exercise, repurchase period Share-Based Compensation Arrangement By Share-based Payment Award, Options, Early Exercise Repurchase Period Share-Based Compensation Arrangement By Share-based Payment Award, Options, Early Exercise Repurchase Period Research Collaboration and License Agreements Collaborative Arrangement Disclosure [Text Block] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Stock-based compensation expense Share-based Payment Arrangement, Noncash Expense Additional Paid-In Capital Additional Paid-in Capital [Member] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Depreciation Depreciation expense Depreciation Remaining extension term Research And Development Option Remaining Extension Term Research And Development Option Remaining Extension Term Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Other income (expense), net Other Nonoperating Income (Expense) Maximum Maximum [Member] Rent expense Operating Leases, Rent Expense, Net Schedule of Milestone Payments by Indications Collaborative Arrangement and Arrangement Other than Collaborative [Table Text Block] Total liabilities Liabilities Japan JAPAN Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Award Type [Axis] Award Type [Axis] Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proof-Of-Concept Trial, CVM Research Proof-Of-Concept Trial, CVM Research Program [Member] Proof-Of-Concept Trial, CVM Research Program Total stockholders' equity Beginning balance Ending balance Stockholders' Equity Attributable to Parent Balance Sheet Related Disclosures [Abstract] Common stock, shares issued (in shares) Common Stock, Shares, Issued Vesting of common stock from early exercises Stock Issued During Period Value Vesting Of Early Exercised Stock Options Stock issued during period value vesting of early exercised stock options. Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Exercisable, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Vested and expected to vest, weighted average exercise price (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Weighted Average Exercise Price Estimated useful life of asset Property, Plant and Equipment, Useful Life Stockholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Non-cash investing and financing activities: Noncash Investing and Financing Items [Abstract] Concentration of Credit and Other Risks Concentration Risk, Credit Risk, Policy [Policy Text Block] Measurement Frequency [Domain] Measurement Frequency [Domain] Proceeds from Open Market Agreement Proceeds From Issuance Open Market Agreement Proceeds From Issuance Open Market Agreement City Area Code City Area Code Share price (in dollars per share) Share Price Accumulated deficit Accumulated deficit Retained Earnings (Accumulated Deficit) Second Indication Second Indication [Member] Second indication. Shares committed under ESPP Share-based Payment Arrangement [Member] Operating lease term Lessee, Operating Lease, Term of Contract Reserve balance for Sales Agreement Reverse Balance For Sales Agreement [Member] Reverse Balance For Sales Agreement Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Issuance of common stock under employee stock purchase plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Schedule of Reconciliation of Restricted Cash Restrictions on Cash and Cash Equivalents [Table Text Block] Income Statement [Abstract] Income Statement [Abstract] Amendment Flag Amendment Flag Asset Class [Domain] Asset Class [Domain] Proceeds from exercise of stock options Proceeds from Stock Options Exercised Short-term marketable securities Short-term Investments Related Party Transaction [Axis] Related Party Transaction [Axis] Equity Components Equity Components [Axis] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Antidilutive Securities Antidilutive Securities [Axis] Total current assets Assets, Current Concentration Risk Type Concentration Risk Type [Domain] Sale of Stock [Domain] Sale of Stock [Domain] Reduction in related party contract asset due to Amended Collaboration Agreement with Merck Increase (Decrease) in Due From Related Parties, Contract Assets, Noncash Increase (Decrease) in Due From Related Parties, Contract Assets, Noncash Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Entity File Number Entity File Number Stock-Based Compensation Share-based Payment Arrangement [Policy Text Block] 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] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Letter of credit as a security deposit Security Deposit Long-Lived Tangible Asset [Domain] Long-Lived Tangible Asset [Domain] Vesting of common stock from early exercises (in shares) Stock Issued During Period Shares Vesting Of Early Exercised Stock Options Stock issued during period shares vesting of early exercised stock options. Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Schedule of Reconciliation of Cash, Cash Equivalent Schedule of Cash and Cash Equivalents [Table Text Block] Entity Small Business Entity Small Business Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Revenue Revenue Benchmark [Member] Weighted-average grant date fair value of stock options, granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Equity Component Equity Component [Domain] Options granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Excluding Shares Issued in Connection with Acquisition, Weighted Average Exercise Price Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Excluding Shares Issued in Connection with Acquisition, Weighted Average Exercise Price Entity Ex Transition Period Entity Ex Transition Period Accrued liabilities Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Statement [Line Items] Statement [Line Items] European Union European Union [Member] Contract liabilities Increase (Decrease) in Contract with Customer, Liability Restricted cash Restricted Cash, Noncurrent Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Transaction price for the two-year performance obligation Revenue, Remaining Performance Obligation, Amount Fair Value Debt Securities, Available-for-sale Short-term marketable securities Short-term Investments [Member] Counterparty Name [Domain] Counterparty Name [Domain] Duration of each purchase period Duration Of Each Purchase Period Duration of each purchase period. Total Operating Leases, Future Minimum Payments Due Option exercise fee Research And Development, Option Exercise Fee Research And Development, Option Exercise Fee Entity Address, State or Province Entity Address, State or Province Purchase of marketable securities Payments to Acquire Marketable Securities Issuance of common stock under 401(k) Plan (in shares) Stock Issued During Period, Shares, Employee Benefit Plan Level 2 Fair Value, Inputs, Level 2 [Member] Long-Lived Tangible Asset [Axis] Long-Lived Tangible Asset [Axis] Research collaboration and license agreements extension fee Research Collaboration And License Agreements Extension Fee Research collaboration and license agreements extension fee. Amortized Cost Debt Securities, Available-for-sale, Amortized Cost Issuance of common stock under Open Market Agreement, net of issuance costs Stock Issued During Period Value Issuance Of Under Open Market Agreement Stock Issued During Period Value Issuance Of Under Open Market Agreement Customer Customer [Axis] Use of Estimates Use of Estimates, Policy [Policy Text Block] Computation of Net Loss Per Common Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Entity Shell Company Entity Shell Company Asset Class [Axis] Asset Class [Axis] Local Phone Number Local Phone Number Summary of Stock Based Compensation Expense Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Forecast Forecast [Member] Deferred rent, current Deferred Rent Credit, Current Research and development performance obligation period Research And Development Performance Obligation Period Research And Development Performance Obligation Period Proceeds from follow on offering, net Proceeds from Issuance of Common Stock Total assets Assets Common stock, shares authorized (in shares) Common Stock, Shares Authorized Geographical [Domain] Geographical [Domain] Numerator: Net Income (Loss) Attributable to Parent [Abstract] Public Stock Offering Public Stock Offering [Member] Public Stock Offering Title of Individual [Axis] Title of Individual [Axis] Remaining extension fee Research Collaboration And License Agreements Remaining Extension Fee Research Collaboration And License Agreements Remaining Extension Fee Research and development Research and Development Expense Balance Sheet Components Supplemental Balance Sheet Disclosures [Text Block] Milestone payments Eligible Milestone Payments Receivable Eligible milestone payments receivable. Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Measurement Frequency [Axis] Measurement Frequency [Axis] Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities Third Indication Third Indication [Member] Third indication. Required amount to expend Amount Of Fund Received For Research Activities, Required Expendable Amount Amount Of Fund Received For Research Activities, Required Expendable Amount Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Entity Tax Identification Number Entity Tax Identification Number Corporate and agency bonds Corporate Bond Securities [Member] Sales Agreement Open Market Sales Agreement [Member] Open Market Sales Agreement [Member] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Axis] Geographical [Axis] Geographical [Axis] Weighted average shares used to compute net loss per share, basic (in shares) Weighted average number of shares used in computing net loss per share, basic (in shares) Weighted Average Number of Shares Outstanding, Basic Net loss per share, diluted (in dollars per share) Earnings Per Share, Diluted Cash on deposit with banks Interest-bearing Deposits in Banks and Other Financial Institutions Vested and expected to vest (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Number of Shares 2022 Operating Leases, Future Minimum Payments, Year One Operating Leases, Future Minimum Payments, Year One Total comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Personnel-related costs Accrued Personnel Expense Current Accrued personnel expense current. Stock options, vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Upfront license fee Upfront License Fee Upfront license fee. Clinical Development Event [Domain] Clinical Development Event [Domain] Clinical Development Event [Domain] Milestone payment for potential achievement of certain clinical development events Milestone Payment For Potential Achievement Of Certain Clinical Development Events Milestone payment for potential achievement of certain clinical development events. Corporate and agency bonds Corporate Debt Securities [Member] Fair Value Measurements Fair Value Disclosures [Text Block] Accounts payable Accounts Payable, Current Concentration Risk Type Concentration Risk Type [Axis] Maximum duration for purchase under employee stock purchase plan Offering Period For Purchase Under Employee Stock Purchase Plan Offering period for purchase under employee stock purchase plan. Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Financial Instruments [Domain] Financial Instruments [Domain] 2021 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Performance of R&D services period Research And Development Service Performance Period Research and development service performance period. Phase 3 Clinical Event Phase 3 Clinical Event [Member] Phase 3 Clinical Event Number of marketable securities in unrealized loss positions less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions Stock repurchase plan, authorized (in shares) Stock Repurchase Program, Number of Shares Authorized to be Repurchased Concentration Risk Benchmark Concentration Risk Benchmark [Domain] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Net unrealized loss on available-for-sale marketable securities OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax Common stock remaining to be sold under open market sales agreement Sale of Stock, Common Stock Available For Issuance Sale of Stock, Common Stock Available For Issuance Number of Options Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Reinvestment amount from option fee Research And Development, Option Exercise Fee, Reinvestment Amount Research And Development, Option Exercise Fee, Reinvestment Amount Property Subject to or Available for Operating Lease, by Major Property Class [Table] Property, Plant, and Equipment, Lessor Asset under Operating Lease [Table] Property and Equipment Property, Plant and Equipment [Table Text Block] Uses and Sources of Liquidity Uses And Sources Of Liquidity, Policy [Policy Text Block] Uses And Sources Of Liquidity, Policy Income Statement Location [Domain] Income Statement Location [Domain] Property and equipment, net Total property and equipment, net Total leasehold improvements Property, Plant and Equipment, Net Total operating expenses Operating Expenses Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Deferred offering costs accrued but not yet paid Deferred Offering Costs Accrued But Not Yet Paid Deferred offering costs accrued but not yet paid. Collaboration Agreement Collaborative Arrangement [Member] Total property and equipment, gross Property, Plant and Equipment, Gross Related Party Transaction [Domain] Related Party Transaction [Domain] Document Quarterly Report Document Quarterly Report Future Minimum Commitments under Unassigned Lease Obligations Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Reduction in letter of credit Reduction In Security Deposit Reduction in security deposit. Beginning balance (in dollars per share) Ending balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Share-based compensation expense Total stock-based compensation expense Share-based Payment Arrangement, Expense Exercisable, weighted average remaining contractual life Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Document Fiscal Period Focus Document Fiscal Period Focus Net loss Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Schedule of Recognized Revenue from Collaboration and License Agreements Schedule Of Recognized Revenue From Collaboration And License Arrangements Table [Table Text Block] Schedule of recognized revenue from collaboration and license arrangements. Amortization of premium (discount) on marketable securities Accretion (Amortization) of Discounts and Premiums, Investments Counterparty Name [Axis] Counterparty Name [Axis] Proceeds from maturities of marketable securities Proceeds from Sale and Maturity of Marketable Securities Construction in process Construction in Progress [Member] 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 Related party contract asset Contract with Customer, Asset, after Allowance for Credit Loss, Current United States UNITED STATES Common stock, par value (in shares) Common Stock, Par or Stated Value Per Share Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Current assets: Assets, Current [Abstract] Total Common Stock, Capital Shares Reserved for Future Issuance Interest income, net Interest Income (Expense), Nonoperating, Net Upfront payment received Collaborative Arrangement Up Front Payment Received Collaborative arrangement up front payment received. Entity Central Index Key Entity Central Index Key Income Taxes Income Tax, Policy [Policy Text Block] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Security Exchange Name Security Exchange Name Revenue Recognition Revenue from Contract with Customer [Policy Text Block] Issuance of common stock upon exercise of stock options (in shares) Options exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Related party receivable from collaboration Increase (Decrease) in Accounts Receivable, Related Parties Weighted average shares used to compute net loss per share, diluted (in shares) Weighted average number of shares used in computing net loss per share, diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Fair Value Measurements, Recurring Fair Value, Recurring [Member] Income Taxes Income Tax Disclosure [Text Block] Cash, cash equivalents and marketable securities Cash, Cash Equivalents, and Short-term Investments Current Fiscal Year End Date Current Fiscal Year End Date Summary of Activity under 2008 Plan and 2018 Plan Share-based Payment Arrangement, Option, Activity [Table Text Block] Merck Original Agreement Merck Original Agreement [Member] Merck Original Agreement Stockholders' Equity Stockholders' Equity Note Disclosure [Text Block] Potentially Dilutive Securities Not Included in the Diluted Net Loss Per Share Calculations Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Fair Value of Financial Instruments Fair Value of Financial Instruments, Policy [Policy Text Block] Total accrued liabilities Accrued Liabilities Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Class of Stock [Line Items] Class of Stock [Line Items] Credit Facility [Axis] Credit Facility [Axis] Net cash (used in) provided by investing activities Net Cash Provided by (Used in) Investing Activities Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Less: accumulated depreciation and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Deferred rent Increase (Decrease) in Prepaid Rent Issuance of common stock under employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] General and administrative General and Administrative Expense [Member] Total current liabilities Liabilities, Current ESPP 401(k) Matching Plan Employee Stock [Member] Aggregate milestone payments Milestone Payment For Potential Achievement Of Certain Clinical Development Events And Certain Regulatory Events Milestone Payment For Potential Achievement Of Certain Clinical Development Events And Certain Regulatory Events Money market funds Money Market Funds [Member] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Title of 12(b) Security Title of 12(b) Security Operating Leased Assets [Line Items] Operating Leased Assets [Line Items] Reimbursable expense Research And Development Reimbursable Expense Research And Development Reimbursable Expense Statement [Table] Statement [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Recent Accounting Pronouncements, Recently Adopted Accounting Pronouncements and Recent Accounting Pronouncements Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Merck Amended Agreement Merck Amended Agreement [Member] Merck Amended Agreement Statistical Measurement [Axis] Statistical Measurement [Axis] Employees and Directors Employees And Directors [Member] Employees and directors. Cover [Abstract] Cover [Abstract] Net purchase of property and equipment Payments to Acquire Machinery and Equipment Vested and expected to vest, weighted average remaining contractual life Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Weighted Average Remaining Contractual Term Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Weighted Average Remaining Contractual Term Additional term Research And Development Option Extend Term Research and development option extend term. Scenario [Domain] Scenario [Domain] 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] Computer equipment Computer Equipment [Member] Proof-Of-Concept Trial, B7H4 Research Program Proof-Of-Concept Trial, B7H4 Research Program [Member] Proof-Of-Concept Trial, B7H4 Research Program Concentration Risk Benchmark Concentration Risk Benchmark [Axis] Milestone payments in connection with potential achievement of certain regulatory events Milestone Payment For Potential Achievement Of Certain Regulatory Events Milestone payment for potential achievement of certain regulatory events. Collaborative Arrangement [Line Items] Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]