| Document and Entity Information | Mar. 29, 2022 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001496323 |
| Document Type | 8-K |
| Document Period End Date | Mar. 29, 2022 |
| Entity Registrant Name | IGM Biosciences, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39045 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | IGMS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
