| Document and Entity Information | Mar. 01, 2022 |
| Cover [Abstract] | |
| Entity Registrant Name | AGENUS INC |
| Security Exchange Name | NASDAQ |
| Amendment Flag | false |
| Entity Central Index Key | 0001098972 |
| Document Type | 8-K |
| Document Period End Date | Mar. 1, 2022 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 000-29089 |
| Entity Tax Identification Number | 06-1562417 |
| 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 |
| Security 12b Title | Common stock, par value $0.01 |
| Trading Symbol | AGEN |
| Entity Emerging Growth Company | false |
