Supplemental Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement [Abstract]
Revenues	$ 97,438	$ 81,360	$ 81,207	$ 67,241	$ 65,932	$ 57,598	$ 59,470	$ 57,480	$ 327,246	$ 240,480	$ 235,278
Income (loss) before income taxes	5,372	8,324	3,506	1,095	(8,243)	(11,455)	(10,189)	6,378	18,297	(23,509)	(98,610)
Income tax expense (benefit)	169	(46)	158	176	(48)	163	191	142	457	448	(20,073)
Net income (loss) from continuing operations	5,203	8,370	3,348	919	(8,195)	(11,618)	(10,380)	6,236	17,840	(23,957)	(78,537)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	2,684	4,759	2,255	3,495	1,154	(1,033)	(2,434)	2,241	13,193	(72)	5,827
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ 2,519	$ 3,611	$ 1,093	$ (2,576)	$ (9,349)	$ (10,585)	$ (7,946)	$ 3,995	$ 4,647	$ (23,885)	$ (108,010)
Earnings (loss) per share:
Income (loss) from continuing operations, basic (in dollars per share)	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ 0.04	$ (0.21)	$ (0.88)
Income (loss) from continuing operations, diluted (in dollars per share)	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ 0.04	$ (0.21)	$ (0.88)
Weighted average common shares outstanding:
Basic (in shares)	117,514	118,359	117,235	113,798	113,939	114,989	114,561	114,281	116,703	114,400	95,532
Diluted (in shares)	123,573	122,708	120,901	113,798	113,939	114,989	114,561	115,663	121,117	114,400	95,532