Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue:
Total revenues	$ 252,954	$ 14,832	$ 275,404	$ 56,906
Operating expenses:
Cost of service revenue	(817)	(911)	(2,589)	(1,545)
Research and development	(42,937)	(32,134)	(125,122)	(107,048)
General and administrative	(21,385)	(14,380)	(54,388)	(39,188)
Contingent purchase price consideration fair value adjustment	(275)	(2,196)	(13,531)	(4,652)
Operating income (loss)	187,540	(34,789)	79,774	(95,527)
Other income (expense):
Gain on extinguishment of debt	6,197		6,197
Loss on modification of debt				(2,720)
Non-operating income (expense)	121	(648)	2,059	(2,040)
Interest expense, net	(16,581)	(16,209)	(49,147)	(44,874)
Net income (loss)	177,277	(51,646)	38,883	(145,161)
Dividends on Series A-1 convertible preferred stock	(53)	(52)	(158)	(157)
Less: net loss attributable to non-controlling interest	(1,275)	(493)	(2,702)	(1,854)
Net income (loss) attributable to Agenus Inc. common stockholders	$ 178,499	$ (51,205)	$ 41,427	$ (143,464)
Per common share data:
Basic net income (loss) attributable to Agenus Inc. common stockholders	$ 0.76	$ (0.28)	$ 0.19	$ (0.87)
Diluted net income (loss) attributable to Agenus Inc. common stockholders	$ 0.72	$ (0.28)	$ 0.18	$ (0.87)
Weighted average number of Agenus Inc. common shares outstanding:
Basic	235,407	182,667	219,677	165,682
Diluted	246,726	182,667	232,512	165,682
Other comprehensive income (loss):
Foreign currency translation gain (loss)	$ (83)	$ 864	$ (2,595)	$ 2,557
Other comprehensive income (loss)	(83)	864	(2,595)	2,557
Comprehensive income (loss)	178,416	(50,341)	38,832	(140,907)
Research and Development [Member]
Revenue:
Total revenues	238,986	4,286	242,265	24,283
Service Revenue [Member]
Revenue:
Total revenues	1,375	1,599	4,201	2,621
Other Revenue [Member]
Revenue:
Total revenues			35	52
Non-Cash Royalty Revenue Related to the Sale of Future Royalties [Member]
Revenue:
Total revenues	$ 12,593	$ 8,947	$ 28,903	$ 29,950