Supplemental Financial Information Supplemental Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Total revenues	$ 121,096	$ 107,101	$ 92,902	$ 106,677	$ 96,345	$ 81,912	$ 81,933	$ 68,329	$ 427,776	$ 328,519	$ 241,515
Income (loss) before income taxes	19,416	10,670	13,644	14,106	5,372	8,324	3,506	1,095	57,836	18,297	(23,509)
Income tax expense (benefit)	(125,210)	141	46	79	169	(46)	158	176	(124,944)	457	448
Net income (loss) from continuing operations	144,626	10,529	13,598	14,027	5,203	8,370	3,348	919
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	2,155	4,006	5,216	4,187	2,684	4,759	2,255	3,495	15,564	13,193	(72)
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ 142,471	$ 6,523	$ 8,382	$ 9,840	$ 2,519	$ 3,611	$ 1,093	$ (2,576)	$ 167,216	$ 4,647	$ (23,885)
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 1.26	$ 0.06	$ 0.07	$ 0.09	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ 1.45	$ 0.04	$ (0.21)
Earnings Per Share, Diluted (in dollars per share)	$ 1.21	$ 0.05	$ 0.07	$ 0.08	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ 1.40	$ 0.04	$ (0.21)
Weighted average common shares outstanding:
Basic (in shares)	113,492	114,969	115,569	115,680	117,514	118,359	117,235	113,798	114,926	116,703	114,400
Diluted (in shares)	118,222	118,801	120,199	122,898	123,573	122,708	120,901	113,798	119,486	121,117	114,400