Consolidated Statements of Changes in Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated Deficit)	Redeemable Non-controlling Interest	Comprehensive Income (Loss)
Balance at Dec. 31, 2008	$ 330,892	$ 375	$ 0	$ 277,464	$ (1,163)	$ 54,216	$ 284,936
Balance, shares at Dec. 31, 2008		37,536,826
Comprehensive income (loss):
Net income (loss), including portion attributable to non-controlling interest	(39,039)							(39,039)
Net income (loss) attributable to redeemable non-controlling interest	16,248						16,248
Net income (loss) attributable to Cowen Group, Inc. stockholders	(55,287)					(55,287)
Defined benefit plans	636				636			636
Foreign currency translation	7				7			7
Total comprehensive income (loss)	(54,644)				643	(55,287)	16,248	(38,396)
Capital contributions	4,906			4,906
Capital distributions	(9,482)					(9,482)
Capital contributions by non-controlling interests in Consolidated Funds							1,613
Capital withdrawals to non-controlling interests in Consolidated Funds							(60,214)
Acquisition of non-controlling interest (see Note 2)	3,112	27		3,085			(13,482)
Common stock issuance upon acquisition (see Note 2)	114,497	151		114,346			1,724
Acquisition of non-controlling interest (see Note 2), shares		2,713,882
Common stock issuance upon acquisition, shares (see Note 2)		15,055,619
Restricted stock awards issued, shares		2,144,138
Issuance of common stock on share offering, shares		17,292,698
Issuance of common stock on share offering	80,607	173		80,434
Amortization of share based compensation	3,637			3,637
Balance at Dec. 31, 2009	473,525	726	0	483,872	(520)	(10,553)	230,825
Balance, shares at Dec. 31, 2009		74,743,163
Comprehensive income (loss):
Net income (loss), including portion attributable to non-controlling interest	(31,690)							(31,690)
Net income (loss) attributable to redeemable non-controlling interest	13,727						13,727
Net income (loss) attributable to Cowen Group, Inc. stockholders	(45,417)					(45,417)
Defined benefit plans	241				241			241
Foreign currency translation	299				299			299
Total comprehensive income (loss)	(44,877)				540	(45,417)	13,727	(31,150)
Capital contributions by non-controlling interests in Consolidated Funds							10,062
Capital withdrawals to non-controlling interests in Consolidated Funds							(105,869)
Restricted stock awards issued, shares		747,046
Amortization of share based compensation	20,608			20,608
Minority interest increase from consolidation of entities							409
Deconsolidation of CHRP GP (see Note 3b)							(1,713)
Deconsolidation of Replication Ltd (see Note 3b)							(3,095)
Balance at Dec. 31, 2010	449,256	726	0	504,480	20	(55,970)	144,346
Balance, shares at Dec. 31, 2010		75,490,209
Comprehensive income (loss):
Net income (loss), including portion attributable to non-controlling interest	(102,183)							(102,183)
Net income (loss) attributable to redeemable non-controlling interest	5,827						5,827
Net income (loss) attributable to Cowen Group, Inc. stockholders	(108,010)					(108,010)
Defined benefit plans	25				25			25
Foreign currency translation	(260)				(260)			(260)
Total comprehensive income (loss)	(108,245)				(235)	(108,010)	5,827	(102,418)
Capital contributions by non-controlling interests in Consolidated Funds							4,038
Capital withdrawals to non-controlling interests in Consolidated Funds							(53,094)
Common stock issuance upon acquisition (see Note 2)	156,048	409		155,639
Common stock issuance upon acquisition, shares (see Note 2)		40,850,133
Restricted stock awards issued, shares		3,053,298
Amortization of share based compensation	28,308			28,308
Minority interest increase from consolidation of entities							3,470
Purchase of treasury stock, at cost	(16,902)		(16,902)
Purchase of treasury stock, at cost, Shares		(5,346,003)
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