Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Computation of earnings per share:
Net income (loss)	$ 55,870	$ 79,879	$ (37,091)
Net Income (Loss) Attributable to Noncontrolling Interest	31,239	37,060	23,791
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	8,442	24,752	7,657
Net income (loss) attributable to Cowen Inc.	24,631	42,819	(60,882)
Preferred stock dividends	6,792	6,792	6,792
Net income (loss) attributable to Cowen Inc. common stockholders	$ 17,839	$ 36,027	$ (67,674)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,525,000	29,545,000	29,492,000
Weighted average shares used in diluted computation, shares	31,286,000	30,735,000	29,492,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.60	$ 1.22	$ (2.29)
Earnings Per Share, Diluted (in dollars per share)	$ 0.57	$ 1.17	$ (2.29)
Quarton
Computation of earnings per share:
Net income (loss) attributable to Cowen Inc.	$ 5,700
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	1,735,000	1,190,000	0
Restricted Stock | Quarton
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	26,000	0	0
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	28,610,357	28,437,860	29,632,020	26,731,289
Common stock, restricted shares, shares	216,912	253,772