Supplemental Financial Information (Details) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
	Dec. 05, 2016	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Quarterly Financial Information Disclosure [Abstract]
Stockholders' Equity Note, Stock Split, Conversion Ratio	0.25
Total revenues		$ 259,931	$ 221,028	$ 234,573	$ 251,384	$ 204,450	$ 178,830	$ 160,530	$ 114,971	$ 966,916	$ 658,781	$ 471,565
Income (loss) before income taxes		4,144	25,861	33,999	34,932	(46,947)	12,109	27,800	14,000	98,936	6,962	(31,487)
Income tax expense (benefit)		(280)	5,083	3,993	6,923	40,646	2,281	(785)	1,911	15,719	44,053	(19,092)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		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)
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares		$ 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) | $ / shares		$ 0.11	$ 0.45	$ 0.12	$ 0.50	$ (2.51)	$ 0.09	$ 0.19	$ 0.05	$ 1.17	$ (2.29)	$ (0.97)
Weighted average common shares outstanding:
Basic (in shares) | shares		29,194	29,610	29,769	29,625	30,934	31,271	28,634	27,061	29,545	29,492	26,857
Diluted (in shares) | shares		30,955	30,844	30,720	30,492	30,934	32,246	29,474	28,401	30,735	29,492	26,857