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Document And Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Mar. 01, 2024
Jun. 30, 2023
Document And Entity Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2023    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Entity Registrant Name IGM Biosciences, Inc.    
Entity Central Index Key 0001496323    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Interactive Data Current Yes    
Current Fiscal Year End Date --12-31    
Entity Filer Category Non-accelerated Filer    
Entity Well-known Seasoned Issuer No    
Entity Public Float     $ 132.1
Entity Shell Company false    
Document Financial Statement Error Correction [Flag] false    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
ICFR Auditor Attestation Flag false    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol IGMS    
Security Exchange Name NASDAQ    
Entity File Number 001-39045    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 77-0349194    
Entity Address, Address Line One 325 E. Middlefield Road    
Entity Address, City or Town Mountain View    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 94043    
City Area Code 650    
Local Phone Number 965-7873    
Document Annual Report true    
Document Transition Report false    
Documents Incorporated by Reference Certain sections of the Registrant’s definitive Proxy Statement to be filed in connection with the Registrant’s 2022 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such definitive Proxy Statement will be filed with the Securities and Exchange Commission pursuant to Regulation 14A within 120 days of the Registrant’s fiscal year ended December 31, 2023.    
Auditor Firm ID 34    
Auditor Name Deloitte & Touche LLP    
Auditor Location San Francisco, California    
Common Stock      
Document And Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   33,286,205  
Non-voting Common Stock      
Document And Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   25,500,383