Consolidated Statements of Changes in Equity (USD $) In Thousands, except Share data	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated Deficit)	Redeemable Non-controlling Interest
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
Net income (loss) attributable to Cowen Group, Inc. stockholders	(45,417)					(45,417)
Net income (loss) attributable to redeemable non-controlling interests	13,727						13,727
Defined benefit plans	241				241
Foreign currency translation	299				299
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(31,150)
Capital withdrawals to non-controlling interests in Consolidated Funds							105,869
Consolidation of funds							409
Deconsolidation of HRP							1,713
Deconsolidation of Replication Ltd							(3,095)
Noncontrolling Interest, Capital Contributions							10,062
Restricted stock awards issued, shares		747,046
Common stock issuance upon close of acquisition (see Note 2)	0
Amortization of share based compensation	20,608			20,608
Balance at Dec. 31, 2010	449,256	726		504,480	20	(55,970)	144,346
Balance, shares at Dec. 31, 2010		75,490,209
Stock Issued During Period, Shares, Acquisitions		40,850,133
Net income (loss) attributable to Cowen Group, Inc. stockholders	(108,010)					(108,010)
Net income (loss) attributable to redeemable non-controlling interests	5,827						5,827
Defined benefit plans	25				25
Foreign currency translation	(260)				(260)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(102,418)
Capital withdrawals to non-controlling interests in Consolidated Funds							53,094
Noncontrolling Interest, Decrease from Deconsolidation	(17,104)
Consolidation of funds							3,470
Noncontrolling Interest, Capital Contributions							4,038
Restricted stock awards issued, shares		3,053,298
Common stock issuance upon close of acquisition (see Note 2)	156,048	409		155,639
Amortization of share based compensation	28,308			28,308
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
Net income (loss) attributable to Cowen Group, Inc. stockholders	(23,885)					(23,885)
Net income (loss) attributable to redeemable non-controlling interests	(72)						(72)
Defined benefit plans	423				423
Foreign currency translation	148				148
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(23,386)
Capital withdrawals to non-controlling interests in Consolidated Funds							(20,729)
Consolidation of funds							18,521
Noncontrolling Interest, Capital Contributions							500
Restricted stock awards issued, shares		4,346,472
Common stock issuance upon close of acquisition (see Note 2)	0
Amortization of share based compensation	24,784			24,784
Purchase of treasury stock, at cost	(14,826)		(14,826)
Purchase of treasury stock, at cost, Shares		(5,946,217)
Balance at Dec. 31, 2012	$ 495,109	$ 1,135	$ (31,728)	$ 713,211	$ 356	$ (187,865)	$ 85,703
Balance, shares at Dec. 31, 2012		112,447,892