Number of shares granted during period (in shares) Issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Document Type Document Type Aggregate Intrinsic Value, Options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Comprehensive loss: Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Options granted (in shares) Number of Options, Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Related Party Related Party [Axis] Collaboration and license agreement, integrin research targets amount (up to) Collaboration And License Agreement Integrin Research Targets Amount Collaboration and license agreement integrin research targets amount. Base rent abated period Lessee, Operating Lease, Base Rent Abatement Period Lessee, Operating Lease, Base Rent Abatement Period Vested (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Additional shares authorized for future grant (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized License Agreements Licensing Agreements [Member] Statistical Measurement Statistical Measurement [Domain] Liabilities and Stockholders’ Equity Liabilities and Equity [Abstract] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Reclassification of restricted stock award liabilities to common stock upon vesting Reclassification Of Restricted Stock Awards From Liabilities To Common Stock Upon Vesting Reclassification of restricted stock awards from liabilities to common stock upon vesting. Security Exchange Name Security Exchange Name Pliant Therapeutics, Inc. Pliant Therapeutics Inc [Member] Pliant Therapeutics, Inc. Loss from operations Operating Income (Loss) Payment of deferred offering costs Payments Of Deferred Offering Costs Represents the payments of deferred offering costs. Total current liabilities Liabilities, Current 2020 Stock Options And Incentive Plan Two Thousand And Twenty Stock Options And Incentive Plan [Member] 2020 Stock options and incentive plan. Underlying common stock fair value (USD per share) Underlying Common Stock Fair Value Underlying common stock fair value. Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Outstanding and unvested, beginning balance (USD per share) Outstanding and unvested, ending balance (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Assets Assets [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Accounting Policies [Abstract] Accounting Policies [Abstract] Vesting of restricted stock awards Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Entity Address, State or Province Entity Address, State or Province Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Accounts payable Accounts Payable, Current Earnings Per Share Reconciliation [Abstract] Earnings Per Share Reconciliation [Abstract] Defined Contribution Plan Defined Contribution Plan [Text Block] Percentage of outstanding equity on fully diluted basis Percentage Of Outstanding Shares On Fully Diluted Basis Percentage of outstanding shares on fully diluted basis. Accrued Liabilities and Other Long-Term Liabilities Accounts Payable and Accrued Liabilities Disclosure [Text Block] Incentive Stock Options and Nonqualified Stock Options Incentive Stock Options And Nonqualified Stock Options [Member] Incentive stock options and nonqualified stock options. Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Accumulated deficit Retained Earnings (Accumulated Deficit) Stock options and ESPP Employee Stock Option And Purchase Plan [Member] Employee stock option and purchase plan. Series C Redeemable Convertible Preferred Stock Series C Redeemable Convertible Preferred Stock [Member] Series C redeemable convertible preferred stock. Number of Options, Outstanding, beginning balance (in shares) Number of Options, Outstanding, ending balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Maturity of short-term investments Proceeds from Maturities, Prepayments and Calls of Short-term Investments Total operating expenses Operating Expenses Novartis Agreement Novartis Agreement [Member] Novartis agreement. Number of Options, Exercisable (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Common stock, par value (USD per share) Common Stock, Par or Stated Value Per Share Leasehold incentive obligation Lease Incentive, Payable, Noncurrent Lease Incentive, Payable, Noncurrent Summary of Other Long-Term Liabilities Other Noncurrent Liabilities [Table Text Block] Other long-term liabilities Total other long-term liabilities Other Liabilities, Noncurrent Stock-based compensation expense APIC, Share-based Payment Arrangement, Increase for Cost Recognition Transaction price of Novartis agreement (up to) Related Party Transaction, Amounts of Transaction Revenue — related party Revenue from Contract with Customer, Excluding Assessed Tax Measurement Frequency Measurement Frequency [Domain] Share price (USD per share) Share Price Third Rock Ventures Third Rock Ventures [Member] Third Rock Ventures. Geographical Geographical [Domain] Expiration period from date of grant Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Statement [Line Items] Statement [Line Items] Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Class of Stock Class of Stock [Domain] Net loss per share, attributable to common stockholders: Earnings Per Share, Basic and Diluted [Abstract] Net unrealized gain/(loss) on short-term investments Unrealized Gain (Loss) on Investments License Agreements [Line Items] License Agreements [Line Items] License agreements. Collaborative Arrangement and Arrangement Other than Collaborative [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Statement [Table] Statement [Table] Statistical Measurement Statistical Measurement [Axis] Novartis Agreement, R&D For Integrin Research Targets Novartis Agreement, R&D For Integrin Research Targets [Member] Novartis Agreement, R&D For Integrin Research Targets Research and Development Arrangement, Contract to Perform for Others, Type Research and Development Arrangement, Contract to Perform for Others, Type [Axis] Number of classes of stock authorized Number Of Classes Of Stock Authorized Number Of Classes Of Stock Authorized Research and Development Arrangement, Contract to Perform for Others, Type Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Expected volatility, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Minimum Fair Value Measurements, Non-recurring Basis Fair Value, Nonrecurring [Member] Entity Small Business Entity Small Business Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Initial public offering, shares issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Fair Value Hierarchy and NAV Fair Value Hierarchy and NAV [Domain] Weighted- Average Remaining Contractual Term, Exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Proceeds from issuances of common stock Proceeds from issuance of common stock Proceeds from Issuance of Common Stock Amendment Flag Amendment Flag Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Total Operating Leases, Future Minimum Payments Due 2021 (remainder of the year) Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Shares used in computing net loss per share attributable to common stockholders: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Shares authorized for future grant (up to) (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 2015 Equity and 2020 Incentive Plan Two Thousand Fifteen And Two Thousand Twenty Equity Incentive Plan [Member] Two thousand fifteen and two thousand twenty equity incentive plan . Proceeds from issuance of common stock upon completion of private placement Proceeds from Issuance of Private Placement Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities Entity Central Index Key Entity Central Index Key Related Party Related Party [Domain] Measurement Frequency Measurement Frequency [Axis] Grant date fair value of stock option Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Fair Value Share based compensation arrangement by share based payment award options grants in period fair value. Related Party Transaction [Line Items] Related Party Transaction [Line Items] Tax credit receivable Income Taxes Receivable, Current Geographical Geographical [Axis] Number of common stock issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period Redeemable Convertible Preferred Stock Redeemable Convertible Preferred Stock [Text Block] Redeemable convertible preferred stock. Research and Development Arrangement, Contract to Perform for Others [Line Items] Research and Development Arrangement, Contract to Perform for Others [Line Items] Adjusted Cost Debt Securities, Available-for-sale, Amortized Cost Lease term extension period Lessee, Operating Lease, Renewal Term Unrecognized compensation cost Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount Temporary Equity, by Class of Stock [Table] Temporary Equity, by Class of Stock [Table] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Accretion of short-term investments Accretion Of Short Term Investments Accretion of short-term investments. Long-Lived Tangible Asset Long-Lived Tangible Asset [Axis] Income Statement Location Income Statement Location [Axis] Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Regent of the University of California License Agreement (the "UC Agreement") License Agreements [Text Block] The entire disclosure of license agreements. Income Statement Location Income Statement Location [Domain] Base rent abated amount Base Rent Abated Amount Base rent abated amount. Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Other non-current assets Other Assets, Noncurrent Plan Name Plan Name [Axis] Deferred offering costs in accounts payable and accrued liabilities Deferred Offering Costs In Accounts Payable And Accrued Liabilities Deferred offering costs in accounts payable and accrued liabilities. Issuance of common/preferred stock Stock Issued During Period, Value, New Issues Liabilities measured at fair value Financial Liabilities Fair Value Disclosure Total assets Assets Related Party Transactions Related Party Transactions Disclosure [Text Block] Outstanding and unvested, beginning balance (in shares) Outstanding and unvested, ending balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Title of 12(b) Security Title of 12(b) Security Summary of Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Grant date fair value of stock option (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Net unrealized gain (loss) on short-term investments Net unrealized gain on short-term investments OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Antidilutive Securities Antidilutive Securities [Axis] Organization and Description of Business Business Description and Basis of Presentation [Text Block] Underwriters Underwriters [Member] Underwriters. Proceeds from the initial public offering Sale of Stock, Consideration Received on Transaction Percentage of noncontrolling ownership Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Product and Service Product and Service [Axis] Total liabilities Liabilities Investment Type Investment Type [Axis] Other Non-current Assets Other Noncurrent Assets [Member] Unrecognized compensation expense, weighted average period Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Conversion of Series A, B, C convertible preferred stock to common stock Temporary Equity Stock Issued During Period Value Conversion Of Convertible Securities Temporary equity stock issued during period value conversion of convertible securities Expected stock price volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Supplemental disclosures of noncash investing and financing activities: Supplemental Cash Flow Elements [Abstract] Weighted- Average Remaining Contractual Term, Vested and expected to vest Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Issuance of Series C redeemable preferred stock, net of issuance costs (in shares) Temporary Equity Stock Issued During Period Shares New Issues Temporary equity stock issued during period shares new issues. Achievement of First Patient Dosing Milestone Achievement Of First Patient Dosing Milestone [Member] Achievement of first patient dosing milestone. Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Net loss attributable to common stockholders Net Income (Loss) Available to Common Stockholders, Basic Level 1 Fair Value, Inputs, Level 1 [Member] Accrued Expenses and Other Liabilities Accrued Expenses And Other Liabilities [Member] Accrued expenses and other liabilities. Adimab Development and Option Agreement (the "Adimab Agreement") Research, Development, and Computer Software Disclosure [Text Block] Accounts payable Increase (Decrease) in Accounts Payable Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Total intrinsic value of outstanding unvested restricted stock awards Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding Dividends paid (USD per share) Common Stock, Dividends, Per Share, Cash Paid Additional Paid-In Capital Additional Paid-in Capital [Member] Antidilutive Securities, Name Antidilutive Securities, Name [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Accrued liabilities Increase (Decrease) in Accrued Liabilities License Agreements [Table] License Agreements [Table] License agreements. Reverse stock split, conversion ratio Stockholders' Equity Note, Stock Split, Conversion Ratio 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] Common stock, shares outstanding (in shares) Balance at beginning of period (in shares) Balance at end of period (in shares) Common Stock, Shares, Outstanding Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] UC Regents U C Regents [Member] UC Regents. Novartis Novartis [Member] Novartis. Research and development Research and development expense Research and Development Expense Initial Public Offering IPO IPO [Member] Amount remaining for achievement Collaboration And License Agreement Integrin Research Targets Amount, Remaining Collaboration And License Agreement Integrin Research Targets Amount, Remaining Weighted-Average Exercise Price per Share, Outstanding, beginning balance (USD per share) Weighted-Average Exercise Price per Share, Outstanding, ending balance (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Common stock, shares authorized (in shares) Common Stock, Shares Authorized Summary of Restricted Stock Activity Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block] Total property and equipment Property, Plant and Equipment, Gross Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Income Statement [Abstract] Income Statement [Abstract] Accumulated Other Comprehensive Gain (Loss) AOCI Attributable to Parent [Member] Reclassification of convertible preferred stock into additional paid in capital upon initial public offering Adjustments to Additional Paid in Capital, Increase in Carrying Amount of Redeemable Preferred Stock Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Number of shares, not purchased (in shares) Share Based Compensation Number Of Restricted Stock Awards Not Purchased Share based compensation number of restricted stock awards, not purchased. Summary of Cash and Cash Equivalents, Money Market Funds and Short-term Investments by Significant Investment Category Cash, Cash Equivalents and Investments [Table Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Current assets Assets, Current [Abstract] Unrealized Gains Debt Securities, Available-for-sale, Unrealized Gain Weighted- Average Exercise Price per Share Share-Based Compensation Arrangement By Share-Based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward] Share-Based Compensation Arrangement By Share-Based Payment Award, Options, Outstanding, Weighted Average Exercise Price Comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Expected dividend Expected dividends Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Net loss per share: Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Rent expenses and common area maintenance charges Operating Lease Rent Expense And Common Area Maintenance Charges Operating lease rent expense and common area maintenance charges. Legal Entity Legal Entity [Axis] Counterparty Name Counterparty Name [Domain] Purchase of property and equipment Payments to Acquire Property, Plant, and Equipment City Area Code City Area Code Document Period End Date Document Period End Date Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] Total shares reserved for future issuance (in shares) Common Stock, Capital Shares Reserved for Future Issuance Convertible preferred stock converted into common shares (in shares) Convertible Preferred Stock, Shares Issued upon Conversion Refundable security deposit Security Deposit Lease term Lessee, Operating Lease, Term of Contract Summary of Components and Classification of Stock-based Compensation Expense for Stock-based Awards Share-based Payment Arrangement, Cost by Plan [Table Text Block] Stock-based compensation expense Share-based Payment Arrangement, Noncash Expense Entity Entity [Domain] Cover [Abstract] Cover [Abstract] Restricted Stock Awards Restricted Stock Awards [Member] Restricted stock awards. Cash and Cash Equivalents Cash and Cash Equivalents [Domain] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] 2022 Operating Leases, Future Minimum Payments Due, Next Twelve Months California CALIFORNIA Market Value Debt Securities, Available-for-sale Equity Components Equity Components [Axis] Unvested restricted shares Restricted Stock [Member] Other long-term liabilities Increase Decrease In Other Long Term Liabilities The increase (decrease) in other long-term liabilities. Share-based Payment Arrangement [Abstract] Share-based Payment Arrangement [Abstract] Minimum Minimum [Member] Balance Sheet Location Balance Sheet Location [Domain] Unrecognized compensation cost Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Underwriting discounts, commissions and offering expenses Expense Related to Distribution or Servicing and Underwriting Fees Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Number of Integrin research targets (up to) Number Of Integrin Research Targets Number of Integrin research targets. Entity Interactive Data Current Entity Interactive Data Current Weighted-Average Exercise Price per Share, Exercised (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Financial Instruments Fair Value Disclosures [Text Block] Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Common Stock Stockholders' Equity Note Disclosure [Text Block] Summary of Option Activity Under the 2015 Plan and 2020 Plan Share-based Payment Arrangement, Option, Activity [Table Text Block] Shares of common stock available for future grants under the 2020 Equity Incentive Plan Two Thousand Twenty Equity Incentive Plan [Member] 2020 equity incentive plan. Additional paid-in capital Additional Paid in Capital, Common Stock Entity Registrant Name Entity Registrant Name Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Deferred rent Accrued Rent, Noncurrent Issuance of Series C redeemable preferred stock, net of issuance costs Temporary Equity, Stock Issued During Period, Value, New Issues Research and development expenses Research and Development Expense [Member] Leased premises under non cancelable operating lease Leased Space Under Non Cancellable Operating Lease Leased space under non cancellable operating lease. Conversion of Series A, B, C convertible preferred stock to common stock Stock Issued During Period, Value, Conversion of Convertible Securities Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Accounts receivable Accounts Receivable, after Allowance for Credit Loss, Current Accrued Liabilities and Other Liabilities [Abstract] Accrued Liabilities and Other Liabilities [Abstract] Research and Development [Abstract] Research and Development [Abstract] Risk-free interest rate, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum Restricted stock awards granted and not purchased Restricted Stock Awards Granted And Not Purchased [Member] Restricted stock awards granted and not purchased. Sale of Stock Sale of Stock [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Unrealized Losses Debt Securities, Available-for-sale, Unrealized Loss Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Total other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Document Transition Report Document Transition Report Adimab Agreement Adimab Agreement [Member] Adimab agreement. Title of Individual Title of Individual [Axis] Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] Collaborative Arrangement and Arrangement Other than Collaborative Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Dividends declared (USD per share) Common Stock, Dividends, Per Share, Declared Level 2 Fair Value, Inputs, Level 2 [Member] Document Quarterly Report Document Quarterly Report Number of Options, Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period General and administrative expenses General and Administrative Expense [Member] Equity [Abstract] Equity [Abstract] Assets measured at fair value Assets, Fair Value Disclosure Additional issuance costs related to initial public offering Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Percentage of common stock shares outstanding Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum Weighted-Average Exercise Price per Share, Granted (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Weighted-average grant date fair value, not purchased (USD per share) Share Based Compensation Restricted Stock Awards Weighted Average Grant Date Fair Value Not Purchased Share based compensation restricted stock awards weighted-average grant date fair value, not purchased. 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 Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Basic (USD per share) Earnings Per Share, Basic Schedule of Research and Development Arrangement, Contract to Perform for Others [Table] Schedule of Research and Development Arrangement, Contract to Perform for Others [Table] Expected term Expected term of options (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Entity File Number Entity File Number Repurchases (USD per share) Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Repurchases Weighted Average Grant Date Fair Value Share based compensation arrangement by share based payment award equity instruments other than options repurchases weighted average grant date fair value. Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Total stock-based compensation expense Share-based Payment Arrangement, Expense Number of votes per share of common stock Number Of Votes Per Share Of Common Stock Number Of Votes Per Share Of Common Stock Balance Sheet Location Balance Sheet Location [Axis] Counterparty Name Counterparty Name [Axis] Common stock, $0.0001 par value; 300,000,000 shares authorized at September 30, 2021 and December 31, 2020; and 35,977,294 and 35,552,795 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively; Common Stock, Value, Issued Novartis Agreement, R&D For PLN-1474 Novartis Agreement, R&D For PLN-1474 [Member] Novartis Agreement, R&D For PLN-1474 Document Fiscal Year Focus Document Fiscal Year Focus Aggregate Intrinsic Value, Exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Cash and Cash Equivalents Cash and Cash Equivalents [Axis] Accrued research and development expenses Accrued Research And Development Expenses Accrued research and development expenses. Entity Current Reporting Status Entity Current Reporting Status Share price of shares issued in initial public offering (USD per share) Sale of Stock, Price Per Share Retirement Benefits [Abstract] Retirement Benefits [Abstract] Related Party Transaction Related Party Transaction [Axis] Vesting of restricted stock awards (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Net loss per share: Net Income (Loss) Per Share [Abstract] Net Income (Loss) Per Share Issued (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Vesting rights percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Novartis Collaboration and License Agreement ("the Novartis Agreement") Collaborative Arrangement Disclosure [Text Block] Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Common stock, shares issued (in shares) Common Stock, Shares, Issued Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Non-employee Director Nonemployee Director [Member] Nonemployee Director. Accounts receivable Increase (Decrease) in Accounts Receivable Balance at beginning of period Balance at end of period Temporary Equity, Carrying Amount, Attributable to Parent Total stockholders’ equity Balance at beginning of period Balance at end of period Stockholders' Equity Attributable to Parent Total liabilities and stockholders’ equity Liabilities and Equity Loss Contingencies [Line Items] Loss Contingencies [Line Items] Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Income Taxes Income Tax Disclosure [Text Block] Entity Address, City or Town Entity Address, City or Town Accrued compensation and benefits Employee-related Liabilities, Current Fair Value Hierarchy and NAV Fair Value Hierarchy and NAV [Axis] Percentage of payroll deductions Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate General and administrative General and Administrative Expense Potentially dilutive securities excluded from computation of diluted net loss per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount 2025 Operating Leases, Future Minimum Payments, Due in Four Years Entity Ex Transition Period Entity Ex Transition Period Indefinite-lived Intangible Assets Indefinite-lived Intangible Assets [Axis] Equity Component Equity Component [Domain] Other non-current assets Increase (Decrease) in Other Noncurrent Assets Property and equipment, net Total property and equipment, net Property, Plant and Equipment, Net Construction-in-progress Construction in Progress [Member] Entity Tax Identification Number Entity Tax Identification Number Equity Incentive Plans and Stock-Based Compensation Share-based Payment Arrangement [Text Block] Net loss Net loss Net income (loss) Net Income (Loss) Attributable to Parent Milestone and annual royalty paid Milestone And Annual Royalty Paid Milestone and annual royalty paid. Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Leases [Abstract] Leases [Abstract] Schedule of Shares Reserved for Future Issuance Schedule Of Common Stock Reserved For Future Issuance Table [Table Text Block] Schedule of common stock reserved for future issuance. Percent of estimated fair value of shares Share Based Compensation Arrangement By Share Based Payment Award Percent Of Estimated Fair Value Of Shares Share Based Compensation Arrangement By Share Based Payment Award Percent Of Estimated Fair Value Of Shares Total current assets Assets, Current Current Fiscal Year End Date Current Fiscal Year End Date Operating expenses: Operating Expenses [Abstract] Net loss per share, attributable to common stockholders: Earnings Per Share [Abstract] Employee stock purchase plan offering period Employee Stock Purchase Plan Offering Period Employee stock purchase plan offering period. Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Shares of common stock available for future issuance under the 2020 ESPP 2020 Employee Stock Purchase Plan Two Thousand And Twenty Employee Stock Purchase Plan [Member] Two thousand and twenty employee stock purchase plan. Options under plan may be granted for periods (up to) Share Based Compensation Options Under Plan May Be Granted For Periods Share based compensation options under plan may be granted for periods. Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders: Weighted-Average Common Shares Outstanding Used To Calculate Net Income (Loss) Per Share Attributable To Common Stockholders [Abstract] Weighted-Average Common Shares Outstanding Used To Calculate Net Income (Loss) Per Share Attributable To Common Stockholders Computer equipment and software Computer Equipment And Software [Member] Computer equipment and software. Lease term extension notice period Lease Term Extension Notice Period Lease term extension notice period. Stockholders’ equity Stockholders' Equity Attributable to Parent [Abstract] Number of Options Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Document Fiscal Period Focus Document Fiscal Period Focus U.S. government agency securities included in cash and cash equivalents and short-term investments US Government Corporations and Agencies Securities [Member] Series A Redeemable Convertible Preferred Stock Series A Redeemable Convertible Preferred Stock [Member] Series A redeemable convertible preferred stock. Aggregate fair value of restricted stock awards vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested Other liabilities — deposits Other Sundry Liabilities, Noncurrent Entity Filer Category Entity Filer Category Common Stock Common Stock [Member] Product and Service Product and Service [Domain] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Long-Lived Tangible Asset Long-Lived Tangible Asset [Domain] Risk-free interest rate, maximum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Restricted stock awards Restricted Common Stock Awards [Member] Restricted common stock awards. Payment made under defined contribution plan Defined Contribution Plan, Cost Schedule of Property and Equipment, Net Property, Plant and Equipment [Table Text Block] Valuation allowance as a percentage on total deferred tax assets Valuation Allowance As Percentage On Deferred Tax Assets Valuation allowance as percentage on deferred tax assets. U.S. Treasury securities included in short-term investments US Treasury Securities [Member] Issuance of common/preferred stock (in shares) Stock Issued During Period, Shares, New Issues Leasehold improvements Leasehold Improvements [Member] Temporary Equity Disclosure [Abstract] Temporary Equity Disclosure [Abstract] Proceeds from upfront non-refundable license fee Proceeds From Upfront Non Refundable License Fee Proceeds from upfront non-refundable license fee. Accrued expenses and other liabilities Deferred Compensation Share-based Arrangements, Liability, Current Conversion of Series A, B, C convertible preferred stock to common stock (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Money Market Funds Money Market Funds [Member] Less: Accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Aggregate Intrinsic Value, Vested and expected to vest Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Option exercises (in shares) Number of Options, Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Related Party Transactions [Abstract] Related Party Transactions [Abstract] Net decrease in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Schedule of Future Minimum Lease Payments Lessee, Operating Lease, Liability, Maturity [Table Text Block] Repurchases (in shares) Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Repurchases In Period The number of equity-based payment instruments, excluding stock (or unit) options, that were repurchased during the reporting period. Percentage of common stock fair market value Common Stock Fair Market Value Percentage Common stock fair market value percentage . Exercises of outstanding stock option awards Stock Options Share-based Payment Arrangement, Option [Member] Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Risk-free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Weighted-Average Exercise Price per Share, Forfeited (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Ownership Ownership [Axis] Other Other Losses Other Losses Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Exercise price as a percentage of estimated grant date fair value of shares Share Based Compensation Arrangement By Share Based Payment Award Exercise Price With Purchase Price Of Common Stock Percentage Of Estimated Grant Date Fair Value For Shareholders Share based compensation arrangement by share based payment award exercise price with purchase price of common stock percentage of estimated grant date fair value for shareholders. Sale of Stock Sale of Stock [Domain] Loss Contingencies [Table] Loss Contingencies [Table] Ownership Ownership [Domain] Plan Name Plan Name [Domain] Purchase of property and equipment in accounts payable and accrued liabilities Purchases Of Property And Equipment Included In Accounts Payable And Accrued Liabilities Purchases of property and equipment included in accounts payable and accrued liabilities. Expected volatility, maximum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Maximum Series B Redeemable Convertible Preferred Stock Series B Redeemable Convertible Preferred Stock [Member] Series B redeemable convertible preferred stock. Proceeds from issuance of common stock upon initial public offering, net of issuance costs Proceeds from Issuance Initial Public Offering 2024 Operating Leases, Future Minimum Payments, Due in Three Years Class of Stock Class of Stock [Axis] Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-sale [Table] Temporary Equity [Line Items] Temporary Equity [Line Items] Accounting Standards Update and Change in Accounting Principle [Table] Accounting Standards Update and Change in Accounting Principle [Table] Title of Individual Title of Individual [Domain] Indefinite-lived Intangible Assets, Major Class Name Indefinite-lived Intangible Assets, Major Class Name [Domain] Accrued liabilities Total accrued liabilities Accrued Liabilities, Current Proceeds from issuance of Series C preferred stock, net of issuance costs Proceeds from Issuance of Preferred Stock and Preference Stock Diluted (USD per share) Earnings Per Share, Diluted 2023 Operating Leases, Future Minimum Payments, Due in Two Years Local Phone Number Local Phone Number Balance at beginning of period (in shares) Balance at end of period (in shares) Temporary Equity, Shares Outstanding Collaborative Arrangement and Arrangement Other than Collaborative Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Depreciation expense Depreciation Entity Address, Address Line One Entity Address, Address Line One Related Party Transaction Related Party Transaction [Domain] Entity Emerging Growth Company Entity Emerging Growth Company Option exercises Stock Issued During Period, Value, Stock Options Exercised Deferred rent Accrued Rent, Current Committed to provide equity financing (up to) Line of Credit Facility, Maximum Borrowing Capacity Award Type Award Type [Axis] Weighted- Average Grant Date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Maximum Maximum [Member] Purchase of short-term investments Payments to Acquire Short-term Investments Conversion of Series A, B, C convertible preferred stock to common stock (in shares) Temporary Equity Stock Issued During Period Shares Conversion Of Convertible Securities Temporary equity stock issued during period shares conversion of convertible securities. Accumulated Deficit Retained Earnings [Member] Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Effective tax rate Effective Income Tax Rate Reconciliation, Percent Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Net cash provided by/(used in) investing activities Net Cash Provided by (Used in) Investing Activities Stock Option Activity, Additional Disclosures Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Weighted-Average Remaining Contractual Term, Options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Tax credit receivable Increase (Decrease) in Income Taxes Receivable Award Type Award Type [Domain] Investments Investments [Domain] 2015 Equity Incentive Plan Two Thousand Fifteen Equity Incentive Plan [Member] 2015 equity incentive plan. Other accrued liabilities Accounts Payable and Other Accrued Liabilities, Current Redeemable Preferred Stock Redeemable Preferred Stock [Member] Corporate debt securities included in cash and cash equivalents and short-term investments Corporate Debt Securities [Member] Trading Symbol Trading Symbol Revenue recognized from related party Revenue from Related Parties Weighted-Average Exercise Price per Share, Exercisable (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Contractual arrangements cancellation period Contractual Arrangements Cancellation Period Contractual arrangements cancellation period. Property and Equipment, net Property, Plant and Equipment Disclosure [Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Private Placement Private Placement [Member] Net Loss Per Share Attributable to Common Stockholders Earnings Per Share [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Laboratory equipment Equipment [Member] Current liabilities Liabilities, Current [Abstract] Entity Shell Company Entity Shell Company Interest and other income (expense), net Interest And Other Income, Net Interest And Other Income, Net Fair Value, Recurring Fair Value, Recurring [Member] Weighted Average Exercise Price per Share, Vested and expected to vest (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price License Agreements [Abstract] License Agreements [Abstract] License agreements. Recently Issued Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Short-term investments Short-term Investments