Quarterly Financial Data (Unaudited) (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Revenue	$ 34,502	$ 19,940	$ 15,715	$ 79,891	$ 6,451	$ 12,802	$ 15,895	$ 1,636	$ 150,048	$ 36,784	$ 42,877
Net income (loss)	(30,851)	(46,277)	(51,867)	17,435	(48,848)	(33,731)	(25,204)	(54,261)
Net income (loss) attributable to Agenus Inc. common shareholders	$ (30,107)	$ (45,526)	$ (50,686)	$ 18,454	$ (47,807)	$ (33,177)	$ (24,723)	$ (54,192)	$ (107,865)	$ (159,899)	$ (120,898)
Basic net income (loss) attributable to Agenus Inc. common stockholders	$ (0.22)	$ (0.33)	$ (0.38)	$ 0.14
Diluted net income (loss) attributable to Agenus Inc. common stockholders	$ (0.22)	$ (0.33)	$ (0.38)	$ 0.12
Basic and diluted net loss attributable to Agenus Inc. common stockholders					$ (0.40)	$ (0.37)	$ (0.24)	$ (0.53)	$ (0.80)	$ (1.44)	$ (1.23)