Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2023	Apr. 03, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K/A
Amendment Flag	true
Document Period End Date	Dec. 31, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Trading Symbol	FUSN
Entity Registrant Name	Fusion Pharmaceuticals Inc.
Entity Central Index Key	0001805890
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		84,865,021
Entity Public Float			$ 302.2
Entity File Number	001-39344
Entity Tax Identification Number	00-0000000
Entity Address, Address Line One	270 Longwood Rd., S.
Entity Address, City or Town	Hamilton
Entity Address, State or Province	ON
Entity Address, Country	CA
Entity Address, Postal Zip Code	L8P 0A6
City Area Code	289
Local Phone Number	799-0891
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Document Annual Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	Z4
Title of 12(b) Security	Common shares, no par value per share
Security Exchange Name	NASDAQ
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE None.
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Boston, Massachusetts
Amendment Description	EXPLANATORY NOTE TO AMENDMENT NO. 1Fusion Pharmaceuticals Inc., or the Company, is filing this Amendment No. 1 on Form 10-K/A, or this Amendment No. 1, to its Annual Report on Form 10-K for the fiscal year ended December 31, 2023, which was filed with the Securities and Exchange Commission on March 20, 2024, or the Original Form 10-K, for the sole purpose of including the information required by Part III of Form 10-K. This information was previously omitted from the Original Form 10-K in reliance on General Instruction G(3) to Form 10-K, which permits the information required by Part III to be incorporated by reference from our definitive proxy statement if such statement is filed no later than 120 days after our fiscal year-end. We are filing this Amendment No. 1 to include Part III information in our Annual Report on Form 10-K because we will not file a definitive proxy statement containing this information within 120 days after the end of the fiscal year covered by the Original Form 10-K. Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, or the Exchange Act, this Amendment No. 1 also contains certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which are attached hereto. Because no financial statements have been included in this Amendment No. 1 and this Amendment No. 1 does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted.Except as explicitly set forth herein, this Amendment No. 1 does not purport to modify or update the disclosures in, or exhibits to, the Original Form 10-K or to update the Original Form 10-K to reflect events occurring after the date of such filing.

Fusion Pharmaceuticals Inc. 10-K/A