Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Computation of earnings per share:
Net income (loss)	$ 6,583	$ 19,598	$ 26,596	$ 75,538
Net Income (Loss) Attributable to Noncontrolling Interest	2,770	4,110	7,188	37,798
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	(6,848)		(4,187)	22,501
Net income (loss) attributable to Cowen Inc.	3,813	15,488	19,408	37,740
Preferred stock dividends	1,698	1,698	5,094	5,094
Net income (loss) attributable to Cowen Inc. common stockholders	$ 2,115	$ 13,790	$ 14,314	$ 32,646
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,529,000	29,610,000	29,687,000	29,662,000
Weighted average shares used in diluted computation, shares	31,264,000	30,844,000	31,381,000	30,613,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.07	$ 0.47	$ 0.48	$ 1.10
Earnings Per Share, Diluted (in dollars per share)	$ 0.07	$ 0.45	$ 0.46	$ 1.07
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	1,735,000	1,234,000	1,694,000	951,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	29,013,137	29,142,547	29,013,137	29,142,547	29,480,287	28,437,860	29,609,472	29,632,020
Common stock, restricted shares, shares	216,912		216,912			253,772