Supplemental Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues
Investment banking									$ 71,762	$ 50,976	$ 38,965
Brokerage									91,167	99,611	112,217
Management fees									38,116	52,466	38,847
Incentive income									5,411	3,265	11,363
Interest and dividends									24,608	22,306	11,547
Reimbursement from affiliates									5,239	4,322	6,816
Other revenues									3,668	1,583	1,936
Consolidated Funds
Interest and dividends, Consolidated Funds									136	569	11,733
Other revenues, Consolidated Funds									373	180	386
Total revenues	65,932	57,598	59,470	57,480	50,395	61,959	58,679	64,245	240,480	235,278	233,810
Income (loss) before income taxes	(8,243)	(11,455)	(10,189)	6,378	(60,336)	(43,858)	4,541	1,043	(23,509)	(98,610)	(53,090)
Income tax expense (benefit)	(48)	163	191	142	(2,353)	71	(17,954)	163	448	(20,073)	(21,400)
Net income (loss) from continuing operations	(8,195)	(11,618)	(10,380)	6,236	(57,983)	(43,929)	22,495	880	(23,957)	(78,537)	(31,690)
Net income (loss) from discontinued operations, net of tax	0	0	0	0	(18,559)	(5,087)	0	0	0	(23,646)	0
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries	1,154	(1,033)	(2,434)	2,241	3,354	(783)	2,458	798	(72)	5,827	13,727
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ (9,349)	$ (10,585)	$ (7,946)	$ 3,995	$ (79,896)	$ (48,233)	$ 20,037	$ 82	$ (23,885)	$ (108,010)	$ (45,417)
Weighted average common shares outstanding:
Basic (in shares)	113,939	114,989	114,561	114,281		115,664	76,330	74,160	114,400	95,532	73,149
Diluted (in shares)	113,939	114,989	114,561	115,663		115,664	77,898	76,083	114,400	95,532	73,149
Earnings (loss) per share:
Income (loss) from continuing operations, basic (in dollars per share)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ (0.54)	$ (0.37)	$ 0.25	$ 0.01	$ (0.21)	$ (0.88)	$ (0.62)
Income (loss) from discontinued operations, basic (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.16)	$ (0.04)	$ 0	$ 0	$ 0.00	$ (0.25)	$ 0.00
Income (loss) from continuing operations, diluted (in dollars per share)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ (0.54)	$ (0.37)	$ 0.26	$ 0.00	$ (0.21)	$ (0.88)	$ (0.62)
Income (loss) from discontinued operations, diluted (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.17)	$ (0.04)	$ 0.00	$ 0.00	$ 0.00	$ (0.25)	$ 0.00