Supplemental Financial Information (Details) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended												12 Months Ended
	Dec. 05, 2016	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, 2016 USD ($) $ / shares shares		Sep. 30, 2016 USD ($) $ / shares shares		Jun. 30, 2016 USD ($) $ / shares shares		Mar. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2017 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares
Quarterly Financial Information Disclosure [Abstract]
Stockholders' Equity Note, Stock Split, Conversion Ratio	0.25
Total revenues		$ 204,450	$ 178,830	$ 160,530	$ 114,971	$ 122,268		$ 131,027		$ 117,231		$ 101,039		$ 658,781		$ 471,565		$ 464,567
Income (loss) before income taxes		(46,947)	12,109	27,800	14,000	(5,037)		24,018		(39,153)		(11,315)		6,962		(31,487)		11,479
Income tax expense (benefit)		40,646	2,281	(785)	1,911	(12,539)		8,759		(11,992)		(3,320)		44,053		(19,092)		(47,496)
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest		(87,593)	9,828	28,585	12,089	7,502		15,259		(27,161)		(7,995)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(11,621)	5,162	21,145	9,105	9,406		18,478		(16,705)		(4,297)		23,791		6,882		15,246
Net income (loss) attributable to Cowen Inc.		(75,972)	4,666	7,440	2,984	(1,904)		(3,219)		(10,456)		(3,698)		(60,882)		(19,277)		43,729
Preferred stock dividends		1,698	1,698	1,698	1,698	1,698		1,698		1,698		1,698		6,792		6,792		4,075
Net income (loss) attributable to Cowen Group, Inc. common stockholders		$ (77,670)	$ 2,968	$ 5,742	$ 1,286	$ (3,602)		$ (4,917)		$ (12,154)		$ (5,396)		$ (67,674)		$ (26,069)		$ 39,654
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares		$ (2.51)	$ 0.09	$ 0.20	$ 0.05	$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ (2.29)	[2]	$ (0.97)	[2]	$ 1.44	[2]
Earnings Per Share, Diluted (in dollars per share) | $ / shares		$ (2.51)	$ 0.09	$ 0.19	$ 0.05	$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ (2.29)	[2]	$ (0.97)	[2]	$ 1.37	[2]
Weighted average common shares outstanding:
Basic (in shares) | shares		30,934	31,271	28,634	27,061	26,973	[1]	26,993	[1]	26,867	[1]	26,591	[1]	29,492	[2]	26,857	[2]	27,522	[2]
Diluted (in shares) | shares		30,934	32,246	29,474	28,401	26,973	[1]	26,993	[1]	26,867	[1]	26,591	[1]	29,492	[2]	26,857	[2]	29,043	[2]

[1] Share and per share amounts have been retroactively updated to reflect the one-for-four reverse stock split effective as of December 5, 2016.

[2] Share and per share amounts have been retroactively updated to reflect the one-for-four reverse stock split effective as of December 5, 2016.