Entity Central Index Key Cover [Abstract] Document Type Document Type 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 Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date 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 Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Entity Public Float Entity Public Float Entity File Number Entity File Number Entity Address, Address Line One Entity Address Address Line1 Entity Address, City or Town Entity Address City Or Town Entity Address, State or Province Entity Address State Or Province Entity Address, Country Entity Address Country Entity Address, Postal Zip Code Entity Address Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Entity Shell Company Entity Shell Company Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Well-known Seasoned Issuer Entity Well Known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Document Annual Report Document Annual Report Document Transition Report Document Transition Report Entity Incorporation, State or Country Code Entity Incorporation State Country Code Title of 12(b) Security Security12b Title Security Exchange Name Security Exchange Name ICFR Auditor Attestation Flag Icfr Auditor Attestation Flag Documents Incorporated by Reference Documents Incorporated By Reference [Text Block] Preferred share tranche right liability. Income taxes payable noncurrent. Non-controlling interest by Parent. Convertible preferred shares. Statement Of Financial Position [Abstract] Statement [Table] Statement [Table] Consolidated Entities Consolidated Entities [Axis] Consolidated Entities Consolidated Entities [Domain] Fusion Pharmaceuticals (Ireland) Limited [Member] Subsidiaries [Member] Statement [Line Items] Statement [Line Items] Assets Assets [Abstract] Current assets: Assets Current [Abstract] Cash and cash equivalents Cash And Cash Equivalents At Carrying Value Short-term investments Short Term Investments Prepaid expenses and other current assets Prepaid Expense And Other Assets Current Restricted cash Restricted Cash Current Total current assets Assets Current Property and equipment, net Property Plant And Equipment Net Deferred tax assets Deferred Income Tax Assets Net Restricted cash Restricted Cash Noncurrent Long-term investments Long Term Investments Other non-current assets Other Assets Noncurrent Total assets Assets Liabilities, Non-Controlling Interest, Convertible Preferred Shares and Shareholders’ Equity (Deficit) Liabilities And Stockholders Equity [Abstract] Current liabilities: Liabilities Current [Abstract] Accounts payable Accounts Payable Current Accrued expenses Accrued Liabilities Current Income taxes payable Accrued Income Taxes Current Deferred revenue Deferred Revenue Current Total current liabilities Liabilities Current Deferred rent, net of current portion Deferred Rent Credit Noncurrent Deferred revenue, net of current portion Deferred Revenue Preferred share tranche right liability Preferred Share Tranche Right Liability Income taxes payable, net of current portion Income Taxes Payable Noncurrent Special voting shares redemption right liability (Notes 2 and 9) Shares Subject To Mandatory Redemption Settlement Terms Amount Noncurrent Total liabilities Liabilities Commitments and contingencies (Note 14) Commitments And Contingencies Non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited (Notes 2 and 9) Non Controlling Interest By Parent Convertible preferred shares, no par value; 0 shares and 132,207,290 shares authorized as of December 31, 2020 and 2019, respectively; 0 shares and 73,125,790 shares issued and outstanding as of December 31, 2020 and 2019, respectively; aggregate liquidation preference of $0 and $77,965 as of December 31, 2020 and 2019, respectively Convertible Preferred Shares Shareholders’ equity (deficit): Stockholders Equity [Abstract] Common shares, no par value, unlimited shares authorized as of December 31, 2020 and 2019; 41,725,797 shares and 1,929,555 shares issued and outstanding as of December 31, 2020 and 2019, respectively Common Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Accumulated other comprehensive income Accumulated Other Comprehensive Income Loss Net Of Tax Accumulated deficit Retained Earnings Accumulated Deficit Total shareholders’ equity (deficit) Stockholders Equity Including Portion Attributable To Noncontrolling Interest Total liabilities, non-controlling interest, convertible preferred shares and shareholders’ equity (deficit) Liabilities And Stockholders Equity Preferred stock, no par value Preferred Stock No Par Value Preferred stock, shares authorized Preferred Stock Shares Authorized Preferred stock, shares issued Preferred Stock Shares Issued Preferred stock, shares outstanding Preferred Stock Shares Outstanding Aggregation liquidity preference Temporary Equity Liquidation Preference Common stock, no par value Common Stock No Par Value Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Change in fair value of preferred share tranche right liability Change in fair value of preferred share warrant liability. Refundable investment tax credits Income Statement [Abstract] Operating expenses: Operating Expenses [Abstract] Research and development Research And Development Expense General and administrative General And Administrative Expense Total operating expenses Operating Expenses Loss from operations Operating Income Loss Other income (expense): Other Nonoperating Income Expense [Abstract] Change in fair value of preferred share tranche right liability Change In Fair Value Of Preferred Share Tranche Right Liability Change in fair value of preferred share warrant liability Change In Fair Value Of Preferred Share Warrant Liability Interest income (expense), net Interest Income Expense Net Refundable investment tax credits Refundable Investment Tax Credits Other income (expense), net Other Nonoperating Income Expense Total other income (expense), net Nonoperating Income Expense Loss before provision for income taxes Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest Income tax provision Income Tax Expense Benefit Net loss Net Income Loss Unrealized gain on investments Unrealized Gain Loss On Investments Comprehensive loss Comprehensive Income Net Of Tax Reconciliation of net loss to net loss attributable to common shareholders: Net Income Loss Available To Common Stockholders Basic [Abstract] Net loss Dividends paid to preferred shareholders in the form of warrants issued Redeemable Preferred Stock Dividends Net loss attributable to common shareholders Net Income Loss Available To Common Stockholders Basic Net loss per share attributable to common shareholders—basic and diluted Earnings Per Share Basic And Diluted Weighted-average common shares outstanding—basic and diluted Weighted Average Number Of Share Outstanding Basic And Diluted Shares of new stock classified as temporary equity issued during the period. Fair value adjustment of preferred share tranche right liability. IssuanceOfCommonSharesUponNetSettlementOfStockOptionExerciseShares. Issuance of warrants to purchase preferred shares as a non-cash dividend to preferred shareholders. Reclassification of tranche right liability upon settlement. Conversion of Class A and B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) into Class A and B convertible preferred share. Conversion of convertible preferred share warrants into common share warrants. Issuance of common stock upon exercise of warrants shares. Statement Of Stockholders Equity [Abstract] Equity Components Statement Equity Components [Axis] Equity Component Equity Component [Domain] Non-Controlling Interest in Fusion Pharmaceuticals (Ireland) Limited [Member] Noncontrolling Interest [Member] Class A and B Convertible Preferred Stock. Class A and B Convertible Preferred Shares [Member] Class A And B Convertible Preferred Stock [Member] Common Shares [Member] Common Stock [Member] Additional Paid-in Capital Additional Paid In Capital [Member] Accumulated Deficit [Member] Retained Earnings [Member] Accumulated Other Comprehensive Income [Member] Accumulated Net Investment Gain Loss Attributable To Noncontrolling Interest [Member] Class of Stock Statement Class Of Stock [Axis] Class of Stock Class Of Stock [Domain] Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right. Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right [Member] Class B Convertible Preferred Shares And Class B Preferred Share Tranche Right [Member] Class B Preferred Exchangeable Shares and Class B Preferred Share Tranche Right. Class B Preferred Exchangeable Shares and Class B Preferred Share Tranche Right [Member] Class B Preferred Exchangeable Shares And Class B Preferred Share Tranche Right [Member] Class B Convertible Preferred Shares and Class B Preferred Exchangeable Shares. Class B Convertible Preferred Shares and Class B Preferred Exchangeable Shares [Member] Class B Convertible Preferred Shares And Class B Preferred Exchangeable Shares [Member] Beginning Balance Beginning Balance (in shares) Shares Outstanding Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs Temporary Equity Stock Issued During Period Value New Issues Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs (in shares) Temporary Equity Stock Issued During Period Shares New Issues Initial fair value of Class B convertible preferred share and preferred exchangeable share tranche right liability Fair Value Adjustment Of Preferred Share Tranche Right Liability Issuance of warrants to purchase Class B convertible preferred shares and Class B preferred exchangeable shares as a non-cash dividend to preferred shareholders Issuance Of Warrants To Purchase Preferred Shares As Non Cash Dividend To Preferred Shareholders Issuance of common shares upon net settlement of stock option exercise Stock Issued During Period Shares Stock Options Exercised Issuance Of Common Shares Upon Net Settlement Of Stock Option Exercise, (Shares) Issuance Of Common Shares Upon Net Settlement Of Stock Option Exercise Shares Reclassification of Class B convertible preferred share and preferred exchangeable share tranche right liability upon settlement Reclassification Of Tranche Right Liability Upon Settlement Conversion of Class A and B preferred exchangeable shares into Class A and B convertible preferred shares Conversion Of Preferred Exchangeable Shares Into Convertible Preferred Shares Conversion of Class A and B preferred exchangeable shares into Class A and convertible preferred shares. Conversion of Class A and B preferred exchangeable shares into Class A and B convertible preferred shares (in shares) Conversion Of Preferred Exchangeable Shares Into Class A And Convertible Preferred Shares Conversion of Class A and B convertible preferred shares into common shares Stock Issued During Period Value Conversion Of Convertible Securities Conversion of Class A and B convertible preferred shares into common shares (in shares) Stock Issued During Period Shares Conversion Of Convertible Securities Conversion of convertible preferred share warrants into common share warrants Conversion Of Convertible Preferred Share Warrants Into Common Share Warrants Issuance of common shares upon closing of initial public offering, net of offering costs and underwriter fees Stock Issued During Period Value New Issues Issuance of common shares upon closing of initial public offering, net of offering costs and underwriter fees (in shares) Stock Issued During Period Shares New Issues Issuance of common shares upon exercise of common share warrants (in shares) Issuance Of Common Stock Upon Exercise Of Warrants Shares Issuance of common shares upon exercise of stock options Stock Issued During Period Value Stock Options Exercised Share-based compensation expense Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value Ending Balance Ending Balance (in shares) Class B convertible preferred shares [Member]. Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right [Member] Class B Convertible Preferred Shares [Member] Issuance of preferred shares and tranche right, issuance costs Payments Of Stock Issuance Costs Issuance of common shares upon closing of initial public offering, offering costs and underwriter fees Noninterest Expense Offering Cost Non-cash rent expense. Proceeds from issuance of Class B convertible preferred shares and Class B preferred share tranche right, net of issuance costs. Proceeds from issuance of Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited and Class B preferred share tranche right, net of issuance costs. Purchases of property and equipment included in accounts payable and accrued expenses. Issuance of warrants to purchase Class B preferred shares and Class B preferred exchangeable shares as a non-cash dividend to preferred shareholders. Issuance of common shares upon net settlement of stock option exercise. Statement Of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided By Used In Operating Activities [Abstract] Net loss Profit Loss 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] Share-based compensation expense Share Based Compensation Depreciation and amortization expense Depreciation Depletion And Amortization Non-cash rent expense Non Cash Rent Expense Change in fair value of preferred share tranche right liability Adjustment Of Warrants Granted For Services Change in fair value of preferred share warrant liability Fair Value Adjustment Of Warrants Amortization of premiums (accretion of discounts) on investments, net Accretion Amortization Of Discounts And Premiums Investments Deferred tax benefit Deferred Income Tax Expense Benefit Changes in operating assets and liabilities: Increase Decrease In Operating Capital [Abstract] Prepaid expenses and other current assets Increase Decrease In Prepaid Deferred Expense And Other Assets Other non-current assets Increase Decrease In Other Noncurrent Assets Accounts payable Increase Decrease In Accounts Payable Accrued expenses Increase Decrease In Accrued Liabilities Deferred revenue Increase Decrease In Deferred Revenue Income taxes payable Increase Decrease In Accrued Income Taxes Payable Net cash used in operating activities Net Cash Provided By Used In Operating Activities Cash flows from investing activities: Net Cash Provided By Used In Investing Activities [Abstract] Purchases of investments Payments To Acquire Investments Maturities of investments Proceeds From Sale Maturity And Collections Of Investments Purchases of property and equipment Payments To Acquire Property Plant And Equipment Net cash used in investing activities Net Cash Provided By Used In Investing Activities Cash flows from financing activities: Net Cash Provided By Used In Financing Activities [Abstract] Proceeds from issuance of Class B convertible preferred shares and Class B preferred share tranche right, net of issuance costs Proceeds From Issance Of Convertible Preferred Stocks Net Of Issuance Cost Proceeds from issuance of Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited and Class B preferred share tranche right, net of issuance costs Proceeds From Issuance Of Exchangeable Shares Tranche Right Net Of Issuance Cost Proceeds from the issuance of common shares upon closing of initial public offering, net of underwriter fees Proceeds From Issuance Initial Public Offering Payment of offering costs Proceeds from issuance of common shares upon exercise of stock options Proceeds From Stock Options Exercised Net cash provided by financing activities Net Cash Provided By Used In Financing Activities Net increase in cash, cash equivalents and restricted cash Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect Cash, cash equivalents and restricted cash at beginning of period Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Cash, cash equivalents and restricted cash at end of period Supplemental disclosure of cash flow information: Supplemental Cash Flow Information [Abstract] Cash paid for income taxes Income Taxes Paid Net Supplemental disclosure of non-cash investing and financing activities: Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract] Purchases of property and equipment included in accounts payable and accrued expenses Purchases Of Property And Equipment Included In Accounts Payable And Accrued Expenses Issuance of warrants to purchase Class B preferred shares and Class B preferred exchangeable shares as a non-cash dividend to preferred shareholders Issuance Of Exchangeable Preferred Shares Issuance of common shares upon net settlement of stock option exercise Issuance Of Common Shares Upon Net Settlement Of Stock Option Exercise Organization Consolidation And Presentation Of Financial Statements [Abstract] Nature of the Business and Basis of Presentation Organization Consolidation And Presentation Of Financial Statements Disclosure [Text Block] Accounting Policies [Abstract] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Collaboration Agreement Collaborative Arrangement Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value Measurements Fair Value Disclosures [Text Block] Investments Debt And Equity Securities [Abstract] Investments Investment [Text Block] PrepaidExpenseAndOtherAssetsCurrentTextBlock Prepaid Expense And Other Assets Current [Abstract] Prepaid Expense and Other Assets Current Prepaid Expense And Other Assets Current [Text Block] Property Plant And Equipment [Abstract] Property and Equipment, Net