Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues
Investment banking	$ 50,976	$ 38,965	$ 10,557
Brokerage	99,611	112,217	17,812
Management fees	52,466	38,847	41,694
Incentive income	3,265	11,363	1,911
Interest and dividends	22,306	11,547	477
Reimbursement from affiliates	4,322	6,816	10,326
Other revenues	1,583	1,936	4,732
Interest and dividends, Consolidated Funds	569	11,733	33,697
Other revenues, Consolidated Funds	180	386	2,695
Total revenues	235,278	233,810	123,901
Expenses
Employee compensation and benefits	203,767	194,919	96,592
Floor brokerage and trade execution	16,475	17,143	2,451
Interest and dividends	8,839	8,971	1,601
Professional, advisory and other fees	33,702	14,547	20,140
Service fees	16,365	15,814	4,452
Communications	16,350	13,972	2,906
Occupancy and equipment	25,673	18,119	11,835
Depreciation and amortization	15,472	11,543	5,761
Client services and business development	16,725	14,470	7,804
Goodwill impairment	7,151
Other	12,354	22,323	12,868
Interest and dividends expense, Consolidated Funds	147	3,078	14,017
Professional, advisory and other fees, Consolidated Funds	2,136	3,094	6,500
Floor brokerage and trade execution, Consolidated Funds		995	707
Other, Consolidated Funds	499	954	2,357
Total expenses	375,655	339,942	189,991
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	15,128	21,980	(2,154)
Bargain purchase gain	22,244
Net realized and unrealized gains (losses) on investments and other transactions, Consolidated Funds	4,925	33,116	55,908
Net realized and unrealized gains (losses) on derivatives, Consolidated Funds	(583)	(761)	(31,750)
Net gains (losses) on foreign currency transactions, Consolidated Funds	53	(1,293)	(3,159)
Total other income (loss)	41,767	53,042	18,845
Income (loss) before income taxes	(98,610)	(53,090)	(47,245)
Income tax expense (benefit)	(20,073)	(21,400)	(8,206)
Net income (loss) from continuing operations	(78,537)	(31,690)	(39,039)
Net income (loss) from discontinued operations, net of tax	(23,646)
Net income (loss)	(102,183)	(31,690)	(39,039)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries	5,827	13,727	16,248
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ (108,010)	$ (45,417)	$ (55,287)
Weighted Average Common Shares Outstanding [Abstract]
Basic (in shares)	95,532	73,149	41,001
Diluted (in shares)	95,532	73,149	41,001
Earnings Per Share [Abstract]
Income (loss) from continuing operations, basic (in dollars per share)	$ (0.88)	$ (0.62)	$ (1.35)
Income (loss) from discontinued operations, net of tax, basic (in dollars per share)	$ (0.25)	$ 0.00	$ 0.00
Income (loss) from continuing operations, diluted (in dollars per share)	$ (0.88)	$ (0.62)	$ (1.35)
Income (loss) from discontinued operations, net of tax, diluted (in dollars per share)	$ (0.25)	$ 0.00	$ 0.00