| Cover | Nov. 09, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 9, 2021 |
| Entity File Number | 000-29089 |
| Entity Registrant Name | AGENUS INC. |
| Entity Central Index Key | 0001098972 |
| Entity Tax Identification Number | 06-1562417 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 3 Forbes Road |
| Entity Address, City or Town | Lexington |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02421 |
| City Area Code | 781 |
| Local Phone Number | 674-4400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.01 par value per share |
| Trading Symbol | AGEN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
