Cash and Cash Equivalents Cash and Cash Equivalents, Unrestricted Cash and Cash Equivalents, Policy [Policy Text Block] Company outstanding shares Company Outstanding Shares Company Outstanding Shares 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 Operating lease right-of-use asset Operating Lease, Right-of-Use Asset Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Personnel-related costs Employee-related Liabilities, Current Income Statement Location [Axis] Income Statement Location [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Related party contract asset Increase (Decrease) in Contract with Customer, Asset Segment and Geographical Information Segment Reporting, Policy [Policy Text Block] Security Exchange Name Security Exchange Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Additional paid-in capital Additional Paid in Capital, Common Stock Weighted average discount rate Operating Lease, Weighted Average Remaining Lease Term Foreign Currency Transactions Foreign Currency Transactions and Translations Policy [Policy Text Block] 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 Entity File Number Entity File Number 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 Options cancelled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period 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. Variable lease costs Variable Lease, Cost Amount of additional fund received for research activities Amount Of Additional Fund Received For Research Activities Amount of additional fund received for research activities Operating lease liability Operating Lease, Weighted Average Discount Rate, Percent 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] 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. Local Phone Number Local Phone Number 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 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, Outstanding, Weighted Average Exercise Price 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 shares authorized; no shares issued or outstanding as of March 31, 2022 and December 31, 2021, 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 Retained Earnings 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] Related Party [Axis] Related Party [Axis] Estimated Useful Life of Asset Schedule Of Estimated Useful Life Of Asset Table [Table Text Block] Schedule of estimated useful life of asset. Property and equipment purchases accrued and not yet paid Capital Expenditures Incurred but Not yet Paid Right of use asset acquired under operating lease on the adoption of ASC 842 Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Maximum percentage of payroll deductions of employee's compensation Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate 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 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 Laboratory equipment and office furniture Laboratory Equipment And Office Furniture [Member] Laboratory equipment and office furniture. Commercial paper Commercial paper Commercial Paper [Member] Operating lease payment obligations Operating Lease, Payments Other non-cash expenses Other Noncash Income (Expense) Research and Development Research and Development Expense, Policy [Policy Text Block] Assets: Assets, Fair Value Disclosure [Abstract] Fair value of financial assets Assets, Fair Value Disclosure Level 1 Fair Value, Inputs, Level 1 [Member] Investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Denominator: Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract] Merck Merck [Member] Merck. Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Entity Small Business Entity Small Business Cash and cash equivalents Cash and Cash Equivalents [Member] Total liabilities and stockholders' equity Liabilities and Equity Accrued expenses Other Accrued Liabilities, Current Contract liabilities Contract with Customer, Liability, Current Options exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price U.S. treasury 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] Entity Interactive Data Current Entity Interactive Data Current Issuance of common stock upon exercise of stock options Stock Issued During Period, Value, Stock Options Exercised Minimum Minimum Minimum [Member] Organization and Description of Business Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] 2023 Lessee, Operating Lease, Liability, to be Paid, Year Two Balance Sheet Location [Domain] Balance Sheet Location [Domain] 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] Manufacturing costs Accrued Manufacturing Costs Accrued Manufacturing costs current. Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding 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 Entity Address, State or Province Entity Address, State or Province 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 Operating lease liability Increase (Decrease) in Operating Lease Liability General and administrative General and Administrative Expense Revision of Prior Period [Axis] Revision of Prior Period [Axis] 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 Issuance of common stock under offering, net of issuance cost (in shares) Stock Issued During Period, Shares, New Issues Present value of lease liabilities Operating Lease, Liability Tenant improvement allowances Tenant Improvements Document Transition Report Document Transition Report Common stock, $0.001 par value; 400,000 shares authorized; 78,087 and 77,962 shares issued and outstanding as of March 31, 2022 and December 31, 2021, respectively Common Stock, Value, Issued Revision of Prior Period, Accounting Standards Update, Adjustment Revision of Prior Period, Accounting Standards Update, Adjustment [Member] Customer Concentration Risk Customer Concentration Risk [Member] Basis of Presentation and Principles of Consolidation Basis of Accounting, Policy [Policy Text Block] Commitments and contingencies Commitments and Contingencies Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Emerging Growth Company Entity Emerging Growth Company 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] Cover [Abstract] Cover [Abstract] 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] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Less: present value adjustment Lessee, Operating Lease, Liability, Undiscounted Excess Amount 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] Number of purchase periods Number Of Purchase Periods Number of purchase periods. Accrued liabilities Accrued Liabilities, Current Related party 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] Document Quarterly Report Document Quarterly Report 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] Operating lease liability, current Operating Lease, Liability, Current 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] Document Fiscal Year Focus Document Fiscal Year Focus 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 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 Previously Reported Previously Reported [Member] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Accounting Standards Update and Change in Accounting Principle Accounting Standards Update and Change in Accounting Principle [Table Text Block] Other (expense) income, net Other Nonoperating Income (Expense) Maximum Maximum [Member] 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] Options granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price 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 Lessee, Operating Lease, Liability, Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Total stockholders' equity Beginning balance Ending balance Stockholders' Equity Attributable to Parent Maximum annual contributions of common stock per employee per year Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount 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 City Area Code City Area Code Exercisable, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Entity Address, City or Town Entity Address, City or Town Estimated useful life of asset Property, Plant and Equipment, Useful Life Stockholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Supplemental disclosures of 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] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Operating lease liability, non-current Operating Lease, Liability, Noncurrent Measurement Frequency [Domain] Measurement Frequency [Domain] Tax benefits realized Share-based Payment Arrangement, Expense, Tax Benefit 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 Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Revision of Prior Period [Domain] Revision of Prior Period [Domain] Entity Filer Category Entity Filer Category Percentage of contribution matched by employer in common stock Defined Contribution Plan, Employer Matching Contribution, Percent of Match Schedule of Reconciliation of Restricted Cash Restrictions on Cash and Cash Equivalents [Table Text Block] Income Statement [Abstract] Income Statement [Abstract] Entity Registrant Name Entity Registrant Name 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] Options cancelled (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Amendment Flag Amendment Flag Share of global development costs percentage Share Of Global Development Costs Percentage Share of global development costs percentage. Equity Components Equity Components [Axis] Entity Tax Identification Number Entity Tax Identification Number Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Antidilutive Securities Antidilutive Securities [Axis] Document Fiscal Period Focus Document Fiscal Period Focus Total current assets Assets, Current Sale of Stock [Domain] Sale of Stock [Domain] Total undiscounted lease payments Lessee, Operating Lease, Liability, to be Paid Concentration Risk Type Concentration Risk Type [Domain] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] 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 Schedule of Related Party Transactions Schedule of Related Party Transactions [Table Text Block] 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. Schedule of Reconciliation of Cash, Cash Equivalent Schedule of Cash and Cash Equivalents [Table Text Block] 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] 2022 (remaining) Lessee, Operating Lease, Liability, to be Paid, Year One Accrued and other 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 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 Retirement Plan Type [Domain] Retirement Plan Type [Domain] 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. Option exercise fee Research And Development, Option Exercise Fee Research And Development, Option Exercise Fee Purchase of marketable securities Payments to Acquire Marketable Securities 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 Customer Customer [Axis] Use of Estimates Use of Estimates, Policy [Policy Text Block] Options grants (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Computation of Net Loss Per Common Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Asset Class [Axis] Asset Class [Axis] Summary of Stock Based Compensation Expense Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Title of 12(b) Security Title of 12(b) Security 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] Title of Individual [Axis] Title of Individual [Axis] Document Type Document Type 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] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Measurement Frequency [Axis] Measurement Frequency [Axis] Net cash used in operating activities 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] Components of Lease Costs Lease, Cost [Table Text Block] Corporate and agency bonds Corporate Bond Securities [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 Common stock reserved for future issuance (in shares) Deferred Compensation Arrangement with Individual, Common Stock Reserved for Future Issuance Non-cash lease expense Noncash lease expense Operating Lease, Right-of-Use Asset, Amortization Expense 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 Stock options, vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 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] AOCI Attributable to Parent AOCI Attributable to Parent [Member] Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Financial Instruments [Domain] Financial Instruments [Domain] Agreed-upon data package Research And Development Collaboration Agreement, Exercisable Agreed-upon Data Package Research And Development Collaboration Agreement, Exercisable Agreed-upon Data Package 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 Document Period End Date Document Period End Date 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 Entity Central Index Key Entity Central Index Key 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 (In Thousands) 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] Total lease cost Lease, Cost 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] Trading Symbol Trading Symbol Collaboration Agreement Collaborative Arrangement [Member] Total property and equipment, gross Property, Plant and Equipment, Gross Related Party Transaction [Domain] Related Party Transaction [Domain] Vested and expected to vest (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number 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 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 Entity Current Reporting Status Entity Current Reporting Status Net loss Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Amortization of premium 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-progress Construction in Progress [Member] Related party contract asset Contract with Customer, Asset, after Allowance for Credit Loss, Current 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 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] Shares reserved for 401(k) (in shares) Total (in shares) Common Stock, Capital Shares Reserved for Future Issuance Interest income, net Interest Income (Expense), Nonoperating, Net Income Taxes Income Tax, Policy [Policy Text Block] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Entity Address, Postal Zip Code Entity Address, Postal Zip Code 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 Related Party [Domain] Related Party [Domain] 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] Entity Address, Address Line One Entity Address, Address Line One Net cash used in 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 Operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] General and administrative General and Administrative Expense [Member] Entity Shell Company Entity Shell Company Total current liabilities Liabilities, Current ESPP 401(k) Matching Plan Employee Stock [Member] Reserve balance for Sales Agreement Reserve Balance For Sales Agreement [Member] Reserve Balance For Sales Agreement Money market funds Money Market Funds [Member] Vested and expected to vest, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Current Fiscal Year End Date Current Fiscal Year End Date 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. Purchases of property and equipment Payments to Acquire Machinery and Equipment Additional term Research And Development Option Extend Term Research and development option extend term. 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] Pension Plan Pension Plan [Member] Operating lease costs Operating Lease, Cost 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] 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, Outstanding, Weighted Average Remaining Contractual Term