Supplemental Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Financial Information [Abstract]
Total revenues	$ 110,611	$ 113,254	$ 119,608	$ 121,094	$ 121,096	$ 107,101	$ 92,902	$ 106,677	$ 464,567	$ 427,776	$ 328,519
Income (loss) before income taxes	(17,781)	(11,083)	13,978	26,365	19,416	10,670	13,644	14,106	11,479	57,836	18,297
Income tax expense (benefit)	(52,708)	(5,081)	3,346	6,947	(125,210)	141	46	79	(47,496)	(124,944)	457
Net income (loss) from continuing operations	34,927	(6,002)	10,632	19,418	144,626	10,529	13,598	14,027
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	4,266	4,344	3,916	2,720	2,155	4,006	5,216	4,187	15,246	15,564	13,193
Preferred Stock Dividends	1,717	1,603	755	0					4,075	0	0
Net income (loss) attributable to Cowen Group, Inc.	30,661	(10,346)	6,716	16,698	$ 142,471	$ 6,523	$ 8,382	$ 9,840	43,729	167,216	4,647
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ 28,944	$ (11,949)	$ 5,961	$ 16,698					$ 39,654	$ 167,216	$ 4,647
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.27	$ (0.11)	$ 0.05	$ 0.15	$ 1.26	$ 0.06	$ 0.07	$ 0.09	$ 0.36	$ 1.45	$ 0.04
Earnings Per Share, Diluted (in dollars per share)	$ 0.26	$ (0.11)	$ 0.05	$ 0.14	$ 1.21	$ 0.05	$ 0.07	$ 0.08	$ 0.34	$ 1.40	$ 0.04
Weighted average common shares outstanding:
Basic (in shares)	107,236	109,191	111,915	112,053	113,492	114,969	115,569	115,680	110,090	114,926	116,703
Diluted (in shares)	112,730	109,191	118,226	118,590	118,222	118,801	120,199	122,898	116,174	119,486	121,117