| Document and Entity Information | Dec. 05, 2020 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 5, 2020 |
| Entity File Number | 0-17999 |
| Entity Registrant Name | ImmunoGen, Inc. |
| Entity Incorporation, State or Country Code | MA |
| Entity Tax Identificatoin Number | 04-2726691 |
| Entity Address, Address Line One | 830 Winter Street |
| Entity Address, State or Province | MA |
| Entity Address, City or Town | Waltham |
| Entity Address, Postal Zip Code | 02451 |
| City Area Code | 781 |
| Local Phone Number | 895-0600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $.01 par value |
| Trading Symbol | IMGN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000855654 |
| Amendment Flag | false |
