Net Income (Loss) - Schedule of Basic And Diluted Net Income (Loss) Per Share (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Per common share data:
Basic	235,407	182,667	219,677	165,682
Effect of potentially dilutive securities:
Diluted	246,726	182,667	232,512	165,682
Net income (loss) attributable to Agenus Inc. per common share:
Basic	$ 178,499,000	$ (51,205,000)	$ 41,427,000	$ (143,464,000)
Agenus Incorporated Inc
Per common share data:
Net income (loss) attributable to Agenus Inc. common stockholders	178,499	(51,205)	41,427	(143,464)
Basic	235,407	182,667	219,677	165,682
Effect of potentially dilutive securities:
Share based compensation awards	7,972		4,428
Convertible preferred stock	3,050		8,407
Warrants	297
Diluted	246,726	182,667	232,512	165,682
Net income (loss) attributable to Agenus Inc. per common share:
Basic	$ 0.76	$ (0.28)	$ 0.19	$ (0.87)
Diluted	$ 0.72	$ (0.28)	$ 0.18	$ (0.87)