Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Percent Impairment related to other-than-temporary Other-than-temporary Impairment Loss, Debt Securities, Available-for-Sale Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Total undiscounted lease payments for the year ending December 31, 2023 Lessee, Operating Lease, Liability, to be Paid Schedule of Cash and Cash Equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Foreign Currency Transactions Foreign Currency Transactions and Translations Policy [Policy Text Block] Net loss Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Entity Address, Postal Zip Code Entity Address, Postal Zip Code Accrued expenses Accrued Expenses Current Accrued expenses current. 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Amount of additional fund received for research activities Amount Of Additional Fund Received For Research Activities Amount of additional fund received for research activities Net loss Net loss Net loss Net losses Net Income (Loss) Attributable to Parent Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Potentially dilutive securities not included in calculations of diluted per share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Percent of stock owned by counterparty Collaborative Arrangement, Percent Of Stock Owned by Counterparty Collaborative Arrangement, Percent Of Stock Owned by Counterparty Equity Component Equity Component [Domain] Expected funding amount Amount Of Funds Expected To Be Received For Research Activities Amount Of Funds Expected To Be Received For Research Activities Scenario [Axis] Scenario [Axis] Assets fair value Assets, Fair Value Disclosure Additions based on tax positions related to current year Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Summary Of Significant Accounting Policies [Line Items] Summary Of Significant Accounting Policies [Line Items] Summary of significant accounting policies. Related Party Related Party [Domain] Common stock options outstanding Common Stock Options Outstanding [Member] Common stock options outstanding. Plan Name Plan Name [Axis] Fair Value Debt Securities, Available-for-Sale Supplemental disclosures of non-cash investing and financing activities: Supplemental Cash Flow Information [Abstract] Options cancelled (in dollars per share) Share-Based Compensation Arrangements by Share-Based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Merck Sharp & Dohme Corp Merck Sharp And Dohme Corporation [Member] Merck Sharp and Dohme Corporation. Net operating loss carryforwards expiration period Net Operating Loss Carryforwards Expiration Period Net operating loss carryforwards expiration period. 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Options cancelled (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Forfeitures in Period Document Annual Report Document Annual Report Total liabilities Liabilities Weighted average discount rate Operating Lease, Weighted Average Remaining Lease Term Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Geographical Geographical [Axis] Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Issuance of common stock under offering, net of issuance costs Stock Issued During Period, Value, New Issues Accrued and other liabilities Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Laboratory equipment and office furniture Laboratory Equipment And Office Furniture [Member] Laboratory equipment and office furniture. Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Fair Value Hierarchy and NAV Fair Value Hierarchy and NAV [Domain] Schedule of Milestone Payments by Indications Collaborative Arrangement and Arrangement Other than Collaborative [Table Text Block] Schedule of Property and Equipment Property, Plant and Equipment [Table Text Block] Revenue Recognition Revenue from Contract with Customer [Policy Text Block] Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Document Type Document Type Weighted Average Exercise Price Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Estimated useful life of asset Property, Plant and Equipment, Useful Life Concentration Risk Benchmark Concentration Risk Benchmark [Domain] Assets: Assets, Fair Value Disclosure [Abstract] Employee Benefit Plan Compensation and Employee Benefit Plans [Text Block] Product and Service Product and Service [Domain] Entity Shell Company Entity Shell Company Related party receivable from collaboration Accounts Receivable, Related Parties, Current Financial Instrument [Axis] Financial Instrument [Axis] Revenue Revenue Benchmark [Member] Document Period End Date Document Period End Date Merck Merck [Member] Merck. Total assets Assets Income Statement Location Income Statement Location [Axis] Antidilutive Securities Antidilutive Securities [Axis] Accrued liabilities Total accrued liabilities Accrued Liabilities, Current Options exercised (in dollars per share) Share-Based Compensation Arrangements by Share-Based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Accounting Policies [Abstract] Accounting Policies [Abstract] Variable lease costs Variable Lease, Cost Total Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Stockholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Foreign tax rate differential Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Common stock options available for grant Common Stock Options Available For Grant [Member] Common stock options available for grant. Customer Customer [Axis] Fair Value Assumptions Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Balance Sheet Components Supplemental Balance Sheet Disclosures [Text Block] Operating lease liability Increase (Decrease) in Operating Lease Liability Short-term Marketable Securities Short-Term Investments [Member] Award Type [Domain] Award Type [Domain] Change in valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent Unrecognized compensation cost expected to be recognized over a weighted-average period Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Exercisable, weighted average remaining contractual life Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Computer equipment Computer Equipment [Member] Entity Registrant Name Entity Registrant Name Issuance of common stock under offering, net of issuance costs (in shares) Issuance of common stock upon initial public offering, net of issuance costs (in shares) Stock Issued During Period, Shares, New Issues Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Summary of significant accounting policies Audit Information [Abstract] Audit Information Expected dividend yield Share-Based Compensation Arrangement by Share-Based Payment Award, Fair Value Assumptions, Expected Dividend Rate Entity Address, City or Town Entity Address, City or Town Tenant improvement allowances Tenant Improvements Percentage of number of shares reserved and available to issue automatically increase outstanding on preceding calendar year Percentage Of Number Of Shares Reserved And Available To Issue Automatically Increase Outstanding On Preceding Calendar Percentage of number of shares reserved and available to issue automatically increase outstanding on preceding calendar. 401(k) matching plan Four Zero One K Matching Plan [Member] Four zero one k matching plan. Related party receivable from collaboration Increase (Decrease) in Accounts Receivable, Related Parties Operating expenses: Operating Expenses [Abstract] Net operating loss carryforwards Deferred Tax Assets, Operating Loss Carryforwards Consolidation Policy Consolidation, Policy [Policy Text Block] Asset Class Asset Class [Domain] Minimum Minimum [Member] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Entity Emerging Growth Company Entity Emerging Growth Company Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Auditor Name Auditor Name 401(k) Pension Plan [Member] Fair Value Measurements Fair Value Disclosures [Text Block] Share price (in dollars per share) Share Price Net operating loss carryforwards Operating Loss Carryforwards Trading Symbol Trading Symbol Entity File Number Entity File Number Research and development Research and Development Expense Additions based on tax positions related to prior year Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Title of Individual Title of Individual [Axis] Total Effective Income Tax Rate Reconciliation, Percent Research Collaboration and License Agreements Research Collaboration and License Agreements [Member] Research Collaboration and License Agreements Concentration Risk Benchmark Concentration Risk Benchmark [Axis] 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 Use of Estimates Use of Estimates, Policy [Policy Text Block] Other comprehensive loss, net of tax: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Sale of common stock, aggregate price Sale Of Stock, Aggregate Offering Price Sale of common stock aggregate price. Clinical Development Event [Axis] Clinical Development Event [Axis] Clinical Development Event Accounts payable Increase (Decrease) in Accounts Payable Percentage of revenue Concentration Risk, Percentage Income Taxes Income Tax Disclosure [Text Block] Related party revenue Revenue from Related Parties 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. Credit Facility Credit Facility [Domain] Net loss per share, basic (in dollars per share) Earnings Per Share, Basic Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Accounts Payable Accounts Payable [Member] Schedule of Restricted Cash and Cash Equivalents Restrictions on Cash and Cash Equivalents [Table Text Block] Concentration Risk Type Concentration Risk Type [Domain] Counterparty Name [Domain] Counterparty Name [Domain] Number of purchase periods Number Of Purchase Periods Number of purchase periods. Total stockholders' equity Beginning balance Ending balance Stockholders' Equity Attributable to Parent Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Lease Contractual Term Lease Contractual Term [Axis] Other temporary differences Deferred Tax Assets, Other Entity Interactive Data Current Entity Interactive Data Current Common stock purchased by related party Sale of Stock, Number of Shares Issued in Transaction First Indication First Indication [Member] First indication. Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Cancellation charges Accrued Manufacturing Costs, Current, Cancellation Charges Accrued Manufacturing Costs, Current, Cancellation Charges Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Numerator: Net Income (Loss) Attributable to Parent [Abstract] Money market funds Money Market Funds [Member] Accumulated Deficit Retained Earnings [Member] Total compensation cost not yet recognized Share-Based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount Total lease costs Lease, Cost Common Stock Common Stock [Member] Maximum percentage of payroll deductions of employee's compensation Share-Based Compensation Arrangement by Share-Based Payment Award, Maximum Employee Subscription Rate Summary of Stock Based Compensation Expense Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Components of Lease Costs Lease, Cost [Table Text Block] Income Taxes Income Tax, Policy [Policy Text Block] IRS Section 174, R&E expenses amortization period Finite-Lived Intangible asset, Useful Life, IRS Section 174 Finite-Lived Intangible asset, Useful Life, IRS Section 174 Marketable Securities Marketable Securities, Policy [Policy Text Block] Statement [Table] Statement [Table] Cash, cash equivalents and short-term marketable securities Cash, Cash Equivalents, and Short-Term Investments Operating lease option additional extend term Lessee, Operating Lease, Renewal Term Letter of Credit Letter of Credit [Member] Current assets: Assets, Current [Abstract] Operating lease liability, non-current Operating Lease, Liability, Noncurrent Research and development credit Research and development tax credits Deferred Tax Assets, Tax Credit Carryforwards, Research Statistical Measurement Statistical Measurement [Axis] Remaining extension term Research And Development Option Remaining Extension Term Research And Development Option Remaining Extension Term Related Party Transaction [Axis] Related Party Transaction [Axis] Leasehold improvements Leasehold Improvements [Member] Percentage of purchase of common stock fair market value Share-Based Compensation Arrangement by Share-Based Payment Award, Discount from Market Price, Offering Date Accumulated deficit Accumulated deficit Retained Earnings (Accumulated Deficit) Purchase of marketable securities Payments to Acquire Marketable Securities Equity Components Equity Components [Axis] Scenario [Domain] Scenario [Domain] Fair Value Measurements, Recurring Fair Value, Recurring [Member] Exercisable, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Weighted Average Exercise Price Segment and Geographical Information Segment Reporting, Policy [Policy Text Block] Comprehensive Loss Comprehensive Income, Policy [Policy Text Block] Corporate Office Space and Laboratory Facility Corporate Office Space And Laboratory Facility [Member] Corporate office space and laboratory facility. Interest income, net Interest Income (Expense), Nonoperating, Net Document Fiscal Year Focus Document Fiscal Year Focus Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Statement [Line Items] Statement [Line Items] Fair Value Hierarchy and NAV Fair Value Hierarchy and NAV [Axis] Proof-Of-Concept Trial, CVM Research Program Proof-Of-Concept Trial, CVM Research Program [Member] Proof-Of-Concept Trial, CVM Research Program Gross Unrealized Loss Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Operating lease right-of-use asset Operating Lease, Right-of-Use Asset Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Auditor Firm ID Auditor Firm ID 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. Deferred tax liabilities: Deferred Tax Liabilities, Gross [Abstract] 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 Rent expense Operating Lease, Expense Document Transition Report Document Transition Report Foreign Foreign Tax Authority [Member] Local Phone Number Local Phone Number Summary of Outstanding Stock Options Share-Based Payment Arrangement, Option, Activity [Table Text Block] Loss from operations Operating Income (Loss) Capitalized R&D Section 174 Deferred Tax Assets, Capitalized Research and Development Costs Deferred Tax Assets, Capitalized Research and Development Costs Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Stock-based compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-Based Compensation Cost Prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets State State and Local Jurisdiction [Member] Denominator: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Common stock, shares outstanding (in shares) Beginning balance (in shares) Ending balance (in shares) Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Long-Lived Tangible Asset Long-Lived Tangible Asset [Axis] 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] Geographical Geographical [Domain] Proceeds from Open Market Agreement, net Proceeds From Issuance Open Market Agreement Proceeds from issuance open market agreement. 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Cash and Cash Equivalents Cash and Cash Equivalents, Unrestricted Cash and Cash Equivalents, Policy [Policy Text Block] Weighted average shares used to compute net loss per share, diluted (in shares) Weighted average number of shares used in calculating net loss per share, diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Less: accumulated depreciation and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Income Tax Disclosure [Line Items] Income Tax Disclosure [Line Items] Income tax disclosure. 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Summary of Available-for-sale Securities Measured at Fair Value on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Exercisable, aggregate intrinsic value Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Intrinsic Value Capitalized Contract Cost Capitalized Contract Cost [Domain] Preferred stock, shares authorized (in shares) Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized 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 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 Foreign Income (Loss) from Continuing Operations before Income Taxes, Foreign Non-qualified stock options with 83(b) election Deferred Tax Liabilities Non Qualified Stock Options Deferred tax liabilities non-qualified stock options. 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Income Statement Location Income Statement Location [Domain] 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] Amendment Flag Amendment Flag Net cash provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Operating lease costs Operating Lease, Cost Issuance of common stock under employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Total comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Share-based Compensation Arrangement by Share-based Payment Award, Options Available For Grant [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options Available For Grant [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options Available For Grant Construction-in-progress Construction in Progress [Member] Gross Unrealized Gain Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax Entity Current Reporting Status Entity Current Reporting Status Other non-current assets Other Assets, Noncurrent Operating loss carryforwards Deferred Tax Assets, Operating Loss Carryforwards, Not Subject to Expiration Collaborative arrangement, review term Collaborative Arrangement, Review Term Collaborative Arrangement, Review Term 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 Letter of credit as a security deposit Security Deposit Percentage of exercise price to fair market value common stock on grant date Share-Based Compensation Arrangement by Share-Based Payment Award, Purchase Price of Common Stock, Percent U.S. federal tax at statutory rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Japan JAPAN Operating lease term Lessee, Operating Lease, Term of Contract Duration of each purchase period Duration Of Each Purchase Period Duration of each purchase period. Issuance of common stock in connection with employee stock purchase plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Option exercise fee Research And Development, Option Exercise Fee Research And Development, Option Exercise Fee Counterparty Name [Axis] Counterparty Name [Axis] Beginning balance (in shares) Ending balance (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number Stock-based compensation (recovery) Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-Based Payment Arrangement, Percent Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Research collaboration and license agreements extension fee Research Collaboration And License Agreements Extension Fee Research collaboration and license agreements extension fee. IPO IPO [Member] 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 Incorporation, State or Country Code Entity Incorporation, State or Country Code State, net of federal benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Commitments And Contingencies Disclosure [Line Items] Commitments And Contingencies Disclosure [Line Items] Commitments and contingencies disclosure. Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] 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. 2019 Employee Stock Purchase Plan Two Thousand Nineteen Employee Stock Purchase Plan [Member] Two thousand nineteen employee stock purchase plan. 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Required amount to expend Amount Of Fund Received For Research Activities, Required Expendable Amount Amount Of Fund Received For Research Activities, Required Expendable Amount Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Open Market Sale Agreement Open Market Sales Agreement [Member] Open Market Sales Agreement. 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Short-term marketable securities Short-Term Investments Commitments And Contingencies Disclosure [Table] Commitments And Contingencies Disclosure [Table] Commitments and contingencies disclosure. Schedule of Reconciliation of Unrecognized Tax Benefits Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block] Personnel-related costs Accrued Personnel Expense Current Accrued personnel expense current. 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Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Sale of Stock Sale of Stock [Axis] Maximum annual contributions of common stock per employee per year Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Asset Class Asset Class [Axis] Net Loss per Share Earnings Per Share, Policy [Policy Text Block] Stock-based compensation expense Share-Based Payment Arrangement, Noncash Expense 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 Common stock reserved for future issuance (in shares) Deferred Compensation Arrangement with Individual, Common Stock Reserved for Future Issuance Additional term Research And Development Option Extend Term Research and development option extend term. Research and development Research and Development Expense [Member] Proof-Of-Concept Trial, B7H4 Research Program Proof-Of-Concept Trial, B7H4 Research Program [Member] Proof-Of-Concept Trial, B7H4 Research Program Computation of Net Loss Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] 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 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. Right to repurchase shares lapses over period Shares Right To Repurchase Lapse Period Shares right to repurchase lapse period. Customer Customer [Domain]