Condensed Consolidated Statements of Changes in Equity (USD $) In Thousands, except Share data	Total	Common Stock [Member]	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated Deficit)	Redeemable Non-controlling Interest
Balance at Dec. 31, 2011	$ 508,465	$ 1,135	$ (16,902)	$ 688,427	$ (215)	$ (163,980)	$ 104,587
Balance, shares at Dec. 31, 2011		114,047,637
Net income (loss) attributable to Cowen Group, Inc. stockholders	(14,536)					(14,536)
Net income (loss) attributable to redeemable non-controlling interests	(1,225)						(1,225)
Defined benefit plans	250				250
Foreign currency translation	126				126
Deconsolidation of funds							(17,104)
Consolidation of funds							18,521
Capital withdrawals to non-controlling interests in Consolidated Funds							(17,268)
Noncontrolling Interest, Increase from Equity Issuance or Sale of Parent Equity Interest							500
Restricted stock awards issued, shares		4,189,400
Amortization of share based compensation	20,578			20,578
Purchase of treasury stock, at cost	(10,904)		(10,904)
Purchase of treasury stock, at cost, Shares		(4,274,148)
Balance at Sep. 30, 2012	$ 503,979	$ 1,135	$ (27,806)	$ 709,005	$ 161	$ (178,516)	$ 88,011
Balance, shares at Sep. 30, 2012		113,962,889