Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Computation of earnings per share:
Net income (loss)	$ 207,064	$ 55,870	$ 79,879
Net Income (Loss) Attributable to Noncontrolling Interest	(9,299)	31,239	37,060
Net income (loss) attributable to Cowen Inc.	216,363	24,631	42,819
Preferred stock dividends	6,792	6,792	6,792
Net income (loss) attributable to Cowen Inc. common stockholders	$ 209,571	$ 17,839	$ 36,027
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	27,790,000	29,525,000	29,545,000
Weighted average shares used in diluted computation, shares	29,519,000	31,286,000	30,735,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 7.54	$ 0.60	$ 1.22
Earnings Per Share, Diluted (in dollars per share)	$ 7.10	$ 0.57	$ 1.17
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	1,644,000	1,735,000	1,190,000
Restricted Stock | Quarton
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	85,000	26,000	0
Restricted Stock Units (RSUs) | Non-employee Director | Equity Plans
Earnings (loss) per share:
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	259,536	216,912
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	26,845,628	28,610,357	28,437,860	29,632,020
Common stock, restricted shares, shares	334,230	216,912