Cover [Abstract] Cover [Abstract] Document Information [Table] Document Information [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Document Information [Line Items] Document Information [Line Items] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Transition Report Document Transition Report Entity Interactive Data Current Entity Interactive Data Current Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Entity File Number Entity File Number Entity Tax Identification Number Entity Tax Identification Number Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Current Fiscal Year End Date Current Fiscal Year End Date Entity Current Reporting Status Entity Current Reporting Status Entity Shell Company Entity Shell Company Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Ex Transition Period Entity Ex Transition Period Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Statement [Line Items] Statement [Line Items] Assets [Abstract] Assets Property, Plant and Equipment, Net Property and equipment, net Property and equipment, net Operating Lease, Right-of-Use Asset Operating lease right-of-use asset Assets, Current [Abstract] Current assets: Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents Cash and cash equivalents, beginning of period Cash and cash equivalents, end of period Cash and cash equivalents Prepaid Expense and Other Assets, Current Prepaid expenses and other current assets Total prepaid expenses and other current assets Other Assets, Current Total current assets Deposits and other longterm assets Non current Deposits and other long-term assets Deposits and other long-term assets Non current. Assets Total assets Liabilities and Equity [Abstract] Liabilities, redeemable convertible preferred shares, redeemable convertible noncontrolling interest and stockholders' equity/members' (deficit) Liabilities, Current [Abstract] Current liabilities: Accounts Payable, Current Accounts payable Accrued Liabilities and Other Liabilities Accrued expenses and other current liabilities Operating Lease, Liability, Current Current portion of operating lease liabilities Liabilities, Current Total current liabilities Operating Lease, Liability, Noncurrent Operating lease liabilities, long-term Liabilities Total liabilities Commitments and Contingencies Commitments and contingencies (Note 8) Redeemable Noncontrolling Interest, Equity, Preferred, Carrying Amount Redeemable convertible preferred shares, no par value; no shares authorized, issued and outstanding at June 30, 2021; 22,851,257 shares authorized, issued and outstanding at December 31, 2020 Carrying Value Redeemable Noncontrolling Interest, Equity, Other, Carrying Amount Redeemable convertible noncontrolling interest Stockholders' Equity Attributable to Parent [Abstract] Stockholders' equity/members' (deficit) Deferred Compensation Equity Incentive shares, no par value; no shares authorized, issued and outstanding at June 30, 2021; 4,312,540 shares authorized and 4,112,012 shares issued and outstanding at December 31, 2020 Retained Earnings (Accumulated Deficit) Accumulated deficit Retained Earnings (Accumulated Deficit) Stockholders' Equity Attributable to Parent Total stockholders' equity/members' (deficit) Ending Balance Beginning Balance Liabilities and Equity Total liabilities, redeemable convertible preferred shares, redeemable convertible noncontrolling interest and members' deficit Preferred Stock, Value, Issued Preferred stock, 10,000,000 shares authorized, $0.0001 par value, no shares issued and outstanding at June 30, 2021; No shares authorized, issued, and outstanding at December 31, 2020 Common Stock, Value, Issued Additional Paid in Capital Additional paid-in-capital Common Class A [Member] Common Class B [Member] Temporary Equity, Par or Stated Value Per Share Temporary Equity, Par or Stated Value Per Share Temporary Equity, Shares Authorized Temporary Equity, Shares Authorized Shares Authorized Common Stock, Par or Stated Value Per Share Common Stock, Par or Stated Value Per Share Common stock par or stated value per share Common Stock, Shares Authorized Common Stock, Shares Authorized Common stock shares authorized Common Stock, Shares, Issued Common Stock, Shares Issued Common Stock, Shares, Outstanding Common Stock, Shares Outstanding Deferred Shares Par Value Per Shares Deferred Shares Par Value Per Shares Deferred Shares Par Value Per Shares. Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance Deferred Compensation Arrangement with Individual, Shares Issued Deferred Compensation Arrangement with Individual, Shares Issued Deferred Shares Outstanding Deferred Shares Outstanding Deferred Shares Outstanding. Temporary Equity, Shares Issued Temporary Equity, Shares Issued Shares Issued Temporary Equity, Shares Outstanding Temporary Equity, Shares Outstanding Temporary equity, shares outstanding Shares Outstanding Preferred Stock, Par or Stated Value Per Share Preferred Stock, Par or Stated Value Per Share Preferred stock par or stated value per share Preferred Stock, Shares Authorized Preferred Stock, Shares Authorized Preferred stock shares authorized Preferred Stock, Shares Issued Preferred Stock, Shares Issued Preferred Stock, Shares Outstanding Preferred Stock, Shares Outstanding Redeemable Convertible Preferred Stock [Member] Income Statement [Abstract] Operating Expenses [Abstract] Operating expenses: Research and Development Expense Research and development General and Administrative Expense General and administrative Operating Expenses Total operating expenses Operating Income (Loss) Loss from operations Interest Expense Interest expense Other Nonoperating Expense Other expense Changes in fair value of derivative tranches liability Changes in fair value of derivative tranche liability Changes in fair value of derivative tranches liability. Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Net loss attributable to redeemable convertible noncontrolling interests Net loss attributable to redeemable convertible noncontrolling interests Redeemable Preferred Stock Dividends Exchange of redeemable noncontrolling interest shares – deemed dividend Earnings Per Share, Basic and Diluted Net loss per share, basic and diluted Weighted Average Number of Shares Outstanding, Basic and Diluted Weighted-average number of common shares used in computing net loss per share, basic and diluted Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss and comprehensive loss Net loss Net loss and comprehensive loss Net Income Loss Available To Common Stockholders Basic Not Affecting Cumulative Losses Net loss attributable to common share members/common stockholders Net loss attributable to common share members/common stockholders Net income loss available to common stockholders basic not affecting cumulative losses. Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Conversion Of Redeemable Noncontrolling Interest To Common Stock Values Conversion of redeemable noncontrolling interest to common stock Conversion of redeemable noncontrolling interest to common stock values. Stock Issued During Period, Shares, Employee Stock Purchase Plans Issuance of incentive shares Stock Issued During Period, Value, Conversion of Units Conversion of redeemable convertible preferred, common, and incentive shares into common stock , Value Stock Issued During Period, Value, New Issues Common stock issued in IPO, net of issuance costs of $16,995 Stock Issued During Period, Value, New Issues Shares, Outstanding Ending Balance (In shares) Beginning Balance (In shares) Conversion Of Redeemable Noncontrolling Interest To Common Stock Shares Conversion of redeemable noncontrolling interest to common stock (In shares) Conversion of redeemable noncontrolling interest to common stock shares. Stock Issued During Period, Shares, New Issues Common stock issued in IPO, net of issuance costs of $16,995 (In shares) Stock issued during period, shares, new issues Stock Issued During Period, Shares, New Issues Issuance of incentive shares Stock Issued During Period, Shares, Issued for Services Cancellations of incentive shares Stock issued during period, shares, issued for services Stock Issued During Period, Shares, Conversion of Convertible Securities Issuance of Series B redeemable convertible preferred shares for cash, net of issuance costs (In shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Stock Issued During Period, Value, Conversion of Convertible Securities Issuance of Series B redeemable convertible preferred shares for cash, net of issuance costs Stock Issued During Period, Value, Conversion of Convertible Securities Stock Issued During Period, Shares, Conversion of Units Conversion of redeemable convertible preferred, common, and incentive shares into common stock (In shares) Stock issued during period, shares, conversion of units APIC, Share-based Payment Arrangement, Increase for Cost Recognition Share-based compensation expenses Net Income (Loss) Attributable to Parent Net loss attributable to Day One Biopharmaceuticals Holding Company, LLC members Net Income (Loss) Available to Common Stockholders, Basic Net loss attributable to common members/ stockholders Equity Components [Axis] Equity Component [Domain] Common Stock [Member] Common Stock [Member] Common shares [Member] Common Shares [Member] Common shares Incentive Shares [Member] Incentive Shares [Member] Incentive shares. Additional Paid-in Capital [Member] Additional Paid-in Capital [Member] Retained Earnings [Member] Accumulated Deficit [Member] Redeemable Noncontrolling Interest [Member] Stock Issuance Costs Stock issuance costs Stock issuance costs. Series B redeemable convertible preferred shares [Member] Series B Redeemable Convertible Preferred Shares. Statement of Cash Flows [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Cash flows from operating activities Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net loss to net cash used in operating activities: Research and Development Asset Acquired Other than Through Business Combination, Written-off Acquired in-process research and development assets Share-based Payment Arrangement, Expense Share-based compensation expense Allocated share based compensation expense Depreciation, Depletion and Amortization Depreciation and amortization expense Operating Lease, Right-of-Use Asset, Amortization Expense Amortization of operating right-of-use assets Amortization of right of use asset operating lease Paid-in-Kind Interest Non-cash interest expense Fair Value Adjustment of Warrants Changes in derivative tranche liabilities Increase (Decrease) in Operating Capital [Abstract] Changes in operating assets and liabilities: Increase (Decrease) in Prepaid Expense and Other Assets Prepaid expenses and other current assets Increase (Decrease) in Deposit Assets Deposits and other long-term assets Increase (Decrease) in Accounts Payable Accounts payable Increase (Decrease) In Accrued Expenses And Other Current Liabilities Accrued expenses and other current liabilities Increase (decrease) in accrued expenses and other current liabilities. Increase Decrease In Operating Lease Liabilities Operating lease liabilities Increase Decrease In Operating Lease Liabilities. Net Cash Provided by (Used in) Operating Activities Net cash used in operating activities Net Cash Provided by (Used in) Investing Activities [Abstract] Cash flows from investing activities Payments to Acquire Property, Plant, and Equipment Purchases of property and equipment Payments To Acquire Process Research and Development Assets Cash paid for acquired in-process research and development assets Payments To Acquire Process Research and Development Assets. Net Cash Provided by (Used in) Investing Activities Cash used in investing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Cash flows from financing activities Proceeds from Issuance of Convertible Preferred Stock Proceeds from issuance of Series B redeemable convertible preferred shares, net of issuance costs Proceeds from Issuance of Common Stock Proceeds from issuance of common stock, net Proceeds from the issuance of common stock Net Cash Provided by (Used in) Financing Activities Net cash provided by financing activities Cash and Cash Equivalents, Period Increase (Decrease) Net increase (decrease) in cash and cash equivalents Supplemental Cash Flow Information [Abstract] Supplemental disclosures of noncash activities Exchange of Preferred, Common, and Incentive Shares in Connection with the Conversion Exchange of 45,331,483 preferred, common, and incentive shares in connection with the Conversion (Note 1) Exchange of preferred, common, and incentive shares in connection with the Conversion. Exchange of Redeemable Convertible Noncontrolling Interest to Shares of Common stock Exchange of redeemable convertible noncontrolling interest to 6,470,382 shares of common stock (Note 13) Exchange of Redeemable Convertible Noncontrolling Interest to Shares of Common Stock. Right of Use Asset Capitalization Right of use asset capitalization Right of Use Asset Capitalization. Deferred Offering Costs Not Yet Paid Deferred offering costs not yet paid Deferred offering costs not yet paid. Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Description of Business, Organization and Liquidity Accounting Policies [Abstract] Significant Accounting Policies [Text Block] Significant Accounting Policies Fair Value Disclosures [Abstract] Fair Value Disclosures [Text Block] Fair Value Measurements Prepaid Expense and Other Assets, Current [Abstract] Prepaid Expenses and Other Current Asset [Text Block] Prepaid Expenses and Other Current Asset Prepaid Expenses and Other Current Asset. Property, Plant and Equipment [Abstract] Property, Plant and Equipment Disclosure [Text Block] Property and Equipment, Net Payables and Accruals [Abstract] Accounts Payable and Accrued Liabilities Disclosure [Text Block] Accrued Expenses and Other Current Liabilities Collaborative Arrangement Disclosure [Text Block] Significant Agreements Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Temporary Equity Disclosure [Abstract] Temporary Equity [Text Block] Redeemable Convertible Preferred Shares Temporary Equity [Text Block]. Stockholders' Equity Note [Abstract] Stockholders' Equity Note Disclosure [Text Block] Common Shares/Common Stock Share-based Payment Arrangement [Text Block] Incentive Shares and Share/Stock-Based Compensation Earnings Per Share [Abstract] Earnings Per Share [Text Block] Net Loss Per Share Redeemable Non Controlling Interest [Text Block] Redeemable NonControlling Interest Disclosure of redeemable non-controlling interest. Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents Deferred Finance Issuance Costs policy text block [Policy Text Block] Deferred Finance Issuance Costs Deferred Finance Issuance Costs policy. Share-based Payment Arrangement [Policy Text Block] Share/Stock-Based Compensation New Accounting Pronouncements, Policy [Policy Text Block] Recently Issued Accounting Pronouncements Use of Estimates, Policy [Policy Text Block] Use of Estimates Basis of Accounting, Policy [Policy Text Block] Basis of Presentation Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis Schedule Of Prepaid And Other Current Assets Table Text Block [Table Text Block] Schedule of Prepaid and Other Current Assets Schedule Of Prepaid And Other Current Assets. Property, Plant and Equipment [Table Text Block] Schedule of Property and Equipment Schedule of Accounts Payable and Accrued Liabilities [Table Text Block] Schedule of Accrued expenses and other current liabilities Lessee, Operating Lease, Liability, Maturity [Table Text Block] Summary of Future Lease Obligations Temporary Equity [Table Text Block] Summary of Series A Redeemable Convertible Preferred Shares Share-based Payment Arrangement [Abstract] Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block] Summary of Stock Option Activity Under The 2021 Plan Schedule of Stock Options Roll Forward [Table Text Block] Summary of The Black-Scholes Option Pricing Model to Estimate The Fair Value of Stock Option Granted Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Summary of Share/Stock-based Compensation Expense Recorded in The Accompanying Condensed Consolidated Statements of Operations and Comprehensive Loss Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of Fair Value of The Incentive Shares Estimated Using An Option Pricing Model Disclosure Details Of Activities Relating To Incentive Shares [Table Text Block] Summary of The Incentive Shares Activity Disclosure details of activities relating to incentive shares. Schedule of Unvested Restricted Stock Units Roll Forward [Table Text Block] Summary of The Unvested Common Stock Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Summary of Basic and Diluted Net Loss Per Share Attributable to Common Shareholders/Stockholders Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Summary of Outstanding Potentially Dilutive Securities Have Been Excluded From Calculation of Diluted Net Loss Per Share OrganizationConsolidationAndPresentationOfFinancialStatementsTable [Table] Organization Consolidation And Presentation Of Financial Statements Table. Sale of Stock [Axis] Sale of Stock [Domain] IPO [Member] Over-Allotment Option [Member] Series A Redeemable Convertible Preferred Stock [Member] Series A Redeemable Convertible Preferred Stock. Related Party [Axis] Related Party [Domain] Millennium Pharmaceuticals, Inc. [Member] Millennium Pharmaceuticals, Inc. Agreement Type [Axis] Agreement Type. Agreement Type [Domain] Agreement Type. The Millennium Stock Exchange Agreement [Member] The Millennium Stock Exchange Agreement. Organization Consolidation And Presentation Of Financial Statements [Line Items] Organization Consolidation And Presentation Of Financial Statements. Payments of Stock Issuance Costs Payments of stock issuance costs Payment of stock issuance costs Conversion of Stock, Shares Issued Number of shares issued upon conversion Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares Share-based compensation arrangement by share-based payment award, options, vested, number of shares Shares Issued, Price Per Share Shares issued, price per share Proceeds from Issuance Initial Public Offering Proceeds from issuance of initial public offering Conversion of Stock, Shares Converted Conversion of stock, shares converted Convertible Preferred Stock, Shares Issued upon Conversion Convertible preferred stock, shares issued upon conversion Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Share-based compensation arrangement by share-based payment award, options, grants in period, net of forfeitures Stockholders' Equity Note, Stock Split Stockholders' equity note, stock split Accounting Policies [Table] Accounting Policies Table. Accounting Policies [Line Items] Accounting Policies LineItems. Cash Equivalents, at Carrying Value Cash equivalents, at carrying value Cash Cash Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Payments Share-based compensation arrangement by share-based payment award, fair value assumptions, expected dividend payments Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net Deferred finance costs, own-share lending arrangement, issuance costs, net Fair Value, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Domain] Fair Value, Recurring [Member] Asset Class [Axis] Asset Class [Domain] Money Market Funds [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Cash and Cash Equivalents, Fair Value Disclosure Money market funds, included in cash and cash equivalents Cash and cash equivalents, fair value disclosure Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Fair value, measurement recurring basis, asset, transfers, net Prepaid Insurance Prepaid insurance Prepaid research and development expenses Prepaid research and development expenses Prepaid Research and Development Expenses. Other Prepaid Expense, Current Other prepaid expenses and other assets Property, Plant and Equipment [Table] Property, Plant and Equipment [Line Items] Property, Plant and Equipment, Gross Property and equipment, gross Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Less: accumulated depreciation and amortization Long-Lived Tangible Asset [Axis] Long-Lived Tangible Asset [Domain] Furniture and Fixtures [Member] Leasehold Improvements [Member] Accrued Payroll Taxes, Current Accrued payroll related expenses Accrued Marketing Costs, Current Accrued issuance costs Other Accrued Liabilities, Current Other Accounts Payable and Accrued Liabilities, Current Total accrued expenses and other current liabilities Accrued research and development expenses Accrued research and development expenses Accrued Research and Development Expenses. Accrued Professional Fees, Current Accrued professional service expenses Collaborative Arrangement and Arrangement Other than Collaborative [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Payment of Milestones Payment of Milestones Payment of Milestones. upfront of cash payment Upfront of cash payment Upfront of Cash Payment. Aggregate cash consideration paid Aggregate cash consideration paid Aggregate Cash Consideration Paid. Upfront Payment Upfront payment Upfront payment. Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Domain] Merck License Agreement [Member] Merck License Agreement. Takeda Asset Agreement [Member] Takeda Asset Agreement. Viracta License Agreement [Member] Income Statement Location [Axis] Income Statement Location [Domain] Research and Development Expense [Member] Statistical Measurement [Axis] Statistical Measurement [Domain] Maximum [Member] Lessee, Operating Lease, Liability, to be Paid, Remainder of Fiscal Year Remaining six months in 2021 Lessee, Operating Lease, Liability, to be Paid, Year One 2022 Lessee, Operating Lease, Liability, to be Paid, Year Two 2023 Lessee, Operating Lease, Liability, to be Paid Total future minimum lease payments Lessee, Operating Lease, Liability, Undiscounted Excess Amount Less: Imputed interest Operating Lease, Liability Present value of operating lease liabilities Lessee, Lease, Description [Table] Lessee, Lease, Description [Line Items] Lessee, Operating Lease, Term of Contract Operating lease term Tenant Improvements Tenant improvement allowances receivable Lessee Operating Lease Incremental Borrowing Rate Lessee operating lease incremental borrowing rate Lessee operating lease incremental borrowing rate. Lessee, Operating Lease, Remaining Lease Term Lessee operating lease renewal lease term Operating Lease, Payments Operating lease payments Loss Contingency Accrual Termination and cancellation charges payable Litigation amount awarded from other party Royalty Fee Payable Royalty fee payable Royalty fee payable. Lease Contractual Term [Axis] Lease Contractual Term [Domain] Lease For Corporate Office Facility [Member] Geographical [Axis] Geographical [Domain] Central America [Member] Agreement [Axis] Agreement [Domain] Research And Development Agreements [Member] Indemnification Agreement [Member] Temporary Equity, by Class of Stock [Table] Temporary Equity [Line Items] Common Stock Shares Issued As A Result Of Conversion Of Temporary Equity Into Permanent Equity Common stock shares issued as a result of conversion of temporary equity into permanent equity Common stock shares issued as a result of conversion of temporary equity into permanent equity. Payment Of Accrued Interest Payment of accrued interest Payment of accrued interest. Temporary Equity Stock Issued During The Period Shares New Issues Temporary equity stock issued during the period shares new issues Temporary equity stock issued during the period shares new issues. Temporary Equity Issue Price Per Share Temporary equity issue price per share Temporary equity issue price per share. Proceeds from Issuance of Redeemable Convertible Preferred Stock Proceeds from redeemable convertible preferred stock Debt Conversion, Converted Instrument, Shares Issued Temporary equity shares issued upon conversion of notes payable Temporary Equity, Shares Subscribed but Unissued Temporary equity shares subscribed but not issued Derivative Liability, Fair Value, Gross Liability Derivative liability at fair value Derivative, Gain (Loss) on Derivative, Net Unrealized gain loss due to change in fair value of derivatives Series B Redeemable Convertible Preferred Stock [Member] Series B Redeemable Convertible Preferred Stock. Details [Axis] Details [Domain] Gross Proceeds [Member] Gross Proceeds. Conditions For Issue [Axis] Conditions For Issue [Domain] Milestone Achievement [Member] Milestone Achievement. Derivative Instrument [Axis] Derivative Contract [Domain] Temporary Equity Related Tranche Liability [Member] Temporary Equity Related Tranche Liability. Basis of Issue [Axis] Basis of Issue [Domain] Milestone Achieved [Member] Milestone Achieved. Temporary Equity, Liquidation Preference Liquidation Value Schedule of Stock by Class [Table] Class of Stock [Line Items] Common Stock Shares Issued Vesting Term Common stock shares issued vesting term Common stock shares issued vesting term. Common Stock Shares Unvested Common stock shares unvested Common stock shares unvested. Number Of Directors Entitled To Be Elected By The Holders Of Common Stock Number of directors entitled to be elected by the holders of common stock Number of directors entitled to be elected by the holders of common stock. Common Stock, Capital Shares Reserved for Future Issuance Common stock shares reserved for future issuance Founder [Member] Founder [Member] Founder One [Member] Founder One [Member] Founder two [Member] Founder Two [Member] Common Shares Purchase Agreement [Member] Common Shares Purchase Agreement [Member] Common Shares Purchase Agreement. Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Class of Stock Sub [Axis] Class Of Stock Sub. Class of Stock Sub [Domain] Class Of Stock Sub. Unvested Common Stock [Member] Unvested Common Stock. Legal Entity [Axis] Entity [Domain] Day One Holding LLC [Member] Day One Holding LLC. Plan Name [Axis] Plan Name [Domain] Incentive Share Plan [Member] Incentive Share Plan. 2021 Stock Incentive Plan [Member] Two thousand and twenty one stock incentive plan. Two Thousand And Twenty One Equity Incentive Plan [Member] 2021 Equity Incentive Plan [Member] Two thousand and twenty one equity incentive plan. Two Thousand And Twenty One Employee Stock Purchase Plan [Member] 2021 Employee Stock Purchase Plan [Member] 2021 employee stock purchase plan. Incremental Shares Reserved For Future Issuance [Member] Incremental Shares Reserved for Future Issuance [Member] Incremental Shares Reserved For Future Issuance. Cliff Vesting [Member] Cliff Vesting [Member] Cliff Vesting. Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Share based compensation by share based payment arrangment number of shares authorized Share Based Compensation By Share Payment Arrangement Intrinsic Value Of Non Vested Outstanding Options Share based compensation by share based arrangement intrinsic value of non vested outstanding options Share based compensation by share payment arrangement intrinsic value of non vested outstanding options. Share Based Compensation By Share Payment Arrangement Weighted Average Remaining Contractual Life Of Non Vested Outstanding Options Share based compensation by share based payment arrangement weighted average remaining contractual life of non vested outstanding options Share based compensation by share payment arrangement weighted average remaining contractual life of non vested outstanding options. Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Share based compensation by share based payment arrangement unrecognized compensation Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Share based compensation by share based payment arrangement unrecognized compensation remaining period for recognition Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Share based compensation by share based payment arrangement term of vesting Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Share based compensation by share based payment award fair value assumptions dividend yield Conversion Of Shares Of One Class Into Another Conversion of shares of one class into another Conversion of shares of one class into another. Additional Number Of Common Stock Shares Reserved For Future Issuance Additional number of common stock shares reserved for future issuance Additional number of common stock shares reserved for future issuance. Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum Additional number of common stock shares reserved for future issuance as percentage of common stock shares outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Share based compensation by share based payment arrangement non vested options granted during the period Granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Share based compensation by share based payment arrangement non vested options granted during the period weighted average grant date fair value Granted Award Type [Axis] Award Type [Domain] Incentive Shares And Stock Options [Member] Incentive Shares and Stock Options [Member] Incentive Shares And Stock Options. Unvested Restricted Stock And Stock Options [Member] Unvested Restricted Stock and Stock Options [Member] Unvested Restricted Stock And Stock Options. Unvested Incentive Shares [Member] Unvested Incentive Shares [Member] Unvested Incentive Shares. Minimum [Member] Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price Participating threshold Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Risk free rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Time to liquidity (in years) Expected term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Grant date fair value Weighted Average Grant Date Fair Value, Granted Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum Risk free rate, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Risk free rate, maximum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Minimum Volatility, minimum Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Maximum Volatility, maximum Share Price Common share fair value Incentive Share Plan Member [Member] Incentive Share Plan Member. Share Based Compensation By Share Based Payment Arrangement Equity Instruments Other Than Options Outstanding Number of Shares, Outstanding at June 30, 2021 Number of Shares, Outstanding at December 31, 2020 Share based compensation by share based payment arrangement equity instruments other than options outstanding. Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Outstanding Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Outstanding Share based compensation arrangement by share based payment award equity instruments other than options outstanding weighted average grant date fair value. Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Forfeited Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Converted To Unvested Common Stock Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Converted to unvested common stock Share based compensation arrangement by share based payment award equity instruments other than options converted to unvested common stock weighted average grant date fair value. Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Number of Shares, Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Number of Shares, Forfeited Share Based Compensation By Share Based Payment Arrangement Equity Instruments Other Than Options Converted To Unvested Common Stock Number of Shares, Converted to unvested common stock Share based compensation by share based payment arrangement equity instruments other than options converted to unvested common stock. Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Number of Shares, Beginning balance Number of Shares, Ending balance Share Based Compensation By Share Based Payment Award Conversion Of Incentive Shares Into Restricted Stock Conversion of incentive shares Share based compensation by share based payment award conversion of incentive shares into restricted stock. Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Beginning balance Weighted Average Grant Date Fair Value, Ending balance Share Based Compensation By Share Based Payment Award Conversion Of Incentive Share Into Restricted Stock Weighted Average Grant Date Fair Value Conversion of incentive shares Share based compensation by share based payment award conversion of incentive share into restricted stock weighted average grant date fair value. Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Vested Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Weighted average grant date fair value, beginning Weighted average grant date fair value, Ending Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number Exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Weighted average grant date fair value, Exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Number of Shares, Beginning balance Number of Shares, Ending balance Share-based Payment Arrangement, Option [Member] Stock options [Member] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] General and Administrative Expense [Member] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Net Income (Loss) Attributable to Noncontrolling Interest Net loss attributable to redeemable convertible noncontrolling interests Temporary Equity, Dividends, Adjustment Exchange of redeemable noncontrolling interest shares – deemed dividend (refer to Note 13) Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Unvested Common Shares [Member] Unvested common shares [Member] Unvested common shares. Redeemable Noncontrolling Interest, by Legal Entity [Table] Related Party Transaction [Axis] Related Party Transaction [Domain] DOT One [Member] DOT -1 [Member] DOT One. Takeda [Member] Takeda. Redeemable Noncontrolling Interest [Line Items] Temporary Equity Shares Converted Into Permanent Equity Temporary equity shares converted into permanent equity Temporary equity shares converted into permanent equity. Adjustment To Additional Paid In Capital Loss On Conversion Of Temporary Equity Into Permanent Equity Adjustment to additional paid in capital loss on conversion of temporary equity into permanent equity Adjustment to additional paid in capital loss on conversion of temporary equity into permanent equity.