v3.25.2
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2025
Jul. 25, 2025
Document and Entity Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Jun. 30, 2025  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Entity Registrant Name IGM Biosciences, Inc.  
Entity Central Index Key 0001496323  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Small Business true  
Entity Emerging Growth Company 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 3 East Third Avenue  
Entity Address, Address Line Two Suite 200  
Entity Address, City or Town San Mateo  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94401  
City Area Code 650  
Local Phone Number 965-7873  
Common Stock    
Document and Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   36,593,204
Non-Voting Common Stock [Member]    
Document and Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   23,699,483