Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Revenues
Investment banking	$ 15,630	$ 14,682
Brokerage	24,013	27,591
Management fees	9,717	11,164
Incentive income	691	4,381
Interest and dividends	5,372	4,559
Reimbursement from affiliates	1,045	1,009
Other revenues	867	690
Interest and dividends, Consolidated Funds	61	169
Other revenues, Consolidated Funds	84	0
Total revenues	57,480	64,245
Expenses
Employee compensation and benefits	46,683	45,087
Floor brokerage and trade execution	3,752	4,110
Interest and dividends	1,724	2,609
Professional, advisory and other fees	3,925	7,140
Service fees	2,237	3,612
Communications	3,401	2,893
Occupancy and equipment	5,242	5,707
Depreciation and amortization	2,155	2,058
Client services and business development	3,826	4,677
Other	3,419	3,710
Interest and dividends, Consolidated Funds	16	46
Professional, advisory and other fees, Consolidated Funds	288	460
Other, Consolidated Funds	71	122
Total expenses	76,739	82,231
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	19,671	17,283
Net realized and unrealized gains (losses) on investments and other transactions, Consolidated Funds	5,964	2,343
Net realized and unrealized gains (losses) on derivatives, Consolidated Funds	40	(441)
Net gains (losses) on foreign currency transactions, Consolidated Funds	(38)	(156)
Total other income (loss)	25,637	19,029
Income (loss) before income taxes	6,378	1,043
Income tax expense (benefit)	142	163
Net income (loss)	6,236	880
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries	2,241	798
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ 3,995	$ 82
Weighted Average Common Shares Outstanding [Abstract]
Basic (in shares)	114,281	74,160
Diluted (in shares)	115,663	76,083
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 0.03	$ 0.00
Diluted (in dollars per share)	$ 0.03	$ 0.00