Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Computation of earnings per share:
Net income (loss)	$ 4,424	$ 20,778	$ 30,006	$ 28,009	$ (87,593)	$ 9,828	$ 28,585	$ 12,089	$ 83,217	$ (37,091)	$ (12,395)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	(655)	5,290	24,607	11,156	(11,621)	5,162	21,145	9,105	40,398	23,791	6,882
Net income (loss) attributable to Cowen Inc.	5,079	15,488	5,399	16,853	(75,972)	4,666	7,440	2,984	42,819	(60,882)	(19,277)
Preferred stock dividends	1,698	1,698	1,698	1,698	1,698	1,698	1,698	1,698	6,792	6,792	6,792
Net income (loss) attributable to Cowen Inc. common stockholders	$ 3,381	$ 13,790	$ 3,701	$ 15,155	$ (77,670)	$ 2,968	$ 5,742	$ 1,286	$ 36,027	$ (67,674)	$ (26,069)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,194,000	29,610,000	29,769,000	29,625,000	30,934,000	31,271,000	28,634,000	27,061,000	29,545,000	29,492,000	26,857,000
Weighted average shares used in diluted computation, shares	30,955,000	30,844,000	30,720,000	30,492,000	30,934,000	32,246,000	29,474,000	28,401,000	30,735,000	29,492,000	26,857,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.12	$ 0.47	$ 0.12	$ 0.51	$ (2.51)	$ 0.09	$ 0.20	$ 0.05	$ 1.22	$ (2.29)	$ (0.97)
Earnings Per Share, Diluted (in dollars per share)	$ 0.11	$ 0.45	$ 0.12	$ 0.50	$ (2.51)	$ 0.09	$ 0.19	$ 0.05	$ 1.17	$ (2.29)	$ (0.97)
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									0	0	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									0	0	0
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									1,190,000	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	28,437,860				29,632,020				28,437,860	29,632,020	26,731,289	26,401,163.000
Common stock, restricted shares, shares	253,772				191,962				253,772	191,962