Supplemental Financial Information (Details) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended																12 Months Ended
	Dec. 05, 2016	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, 2015 USD ($) $ / shares shares		Sep. 30, 2015 USD ($) $ / shares shares		Jun. 30, 2015 USD ($) $ / shares shares		Mar. 31, 2015 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares		Dec. 31, 2014 USD ($) $ / shares shares
Quarterly Financial Information Disclosure [Abstract]
Stockholders' Equity Note, Stock Split, Conversion Ratio	0.25
Total revenues		$ 122,268		$ 131,027		$ 117,231		$ 101,039		$ 110,611		$ 113,254		$ 119,608		$ 121,094		$ 471,565		$ 464,567		$ 427,776
Income (loss) before income taxes		(5,037)		24,018		(39,153)		(11,315)		(17,781)		(11,083)		13,978		26,365		(31,487)		11,479		57,836
Income tax expense (benefit)		(12,539)		8,759		(11,992)		(3,320)		(52,708)		(5,081)		3,346		6,947		(19,092)		(47,496)		(124,944)
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest		7,502		15,259		(27,161)		(7,995)		34,927		(6,002)		10,632		19,418
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		9,406		18,478		(16,705)		(4,297)		4,266		4,344		3,916		2,720		6,882		15,246		15,564
Net income (loss) attributable to Cowen Group, Inc.		(1,904)		(3,219)		(10,456)		(3,698)		30,661		(10,346)		6,716		16,698		(19,277)		43,729		167,216
Preferred stock dividends		1,698		1,698		1,698		1,698		1,717		1,603		755		0		6,792		4,075		0
Net income (loss) attributable to Cowen Group, Inc. common stockholders		$ (3,602)		$ (4,917)		$ (12,154)		$ (5,396)		$ 28,944		$ (11,949)		$ 5,961		$ 16,698		$ (26,069)		$ 39,654		$ 167,216
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares		$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ 1.08	[1]	$ (0.44)	[1]	$ 0.21	[1]	$ 0.60	[1]	$ (0.97)	[2],[3]	$ 1.44	[2],[3]	$ 5.82	[2],[3]
Earnings Per Share, Diluted (in dollars per share) | $ / shares		$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ 1.03	[1]	$ (0.44)	[1]	$ 0.20	[1]	$ 0.56	[1]	$ (0.97)	[2],[3]	$ 1.37	[2],[3]	$ 5.60	[2],[3]
Weighted average common shares outstanding:
Basic (in shares) | shares		26,973	[1]	26,993	[1]	26,867	[1]	26,591	[1]	26,809	[1]	27,297	[1]	27,978	[1]	28,013	[1]	26,857	[3]	27,522	[3]	28,731	[2],[3]
Diluted (in shares) | shares		26,973	[1]	26,993	[1]	26,867	[1]	26,591	[1]	28,182	[1]	27,297	[1]	29,556	[1]	29,647	[1]	26,857	[2],[3]	29,043	[2],[3]	29,871	[2],[3]

[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.

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