Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Computation of earnings per share:
Net income (loss)	$ (72,111)	$ 10,295
Net Income (Loss) Attributable to Noncontrolling Interest	(62,188)	512
Net income (loss) attributable to Cowen Inc.	(9,923)	9,783
Preferred stock dividends	1,698	1,698
Net income (loss) attributable to Cowen Inc. common stockholders	$ (11,621)	$ 8,085
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	28,598,000	29,750,000
Weighted average shares used in diluted computation, shares	28,598,000	31,625,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ (0.41)	$ 0.27
Earnings Per Share, Diluted (in dollars per share)	$ (0.41)	$ 0.26
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	1,875,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	27,628,117	29,580,687	28,610,357	28,437,860
Common stock, restricted shares, shares	299,436		216,912