Condensed Consolidated Statements of Changes in Equity (USD $)		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, 2012		$ 495,109,000		$ 1,135,000	$ (31,728,000)	$ 713,211,000	$ 356,000	$ (187,865,000)	$ 85,703,000
Common Stock, Shares, Outstanding at Dec. 31, 2012				112,447,892
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders		(1,483,000)						(1,483,000)
Net income (loss) attributable to redeemable non-controlling interests		5,750,000							5,750,000
Defined benefit plans		(242,000)	[1]				(242,000)
Foreign currency translation		(4,000)	[1]				(4,000)
Capital contributions									10,181,000
Capital withdrawals									(10,072,000)
Restricted stock awards issued, shares				3,937,337
Common stock issued upon acquisition (See Note 2)		6,297,000		25,000		6,272,000
Common stock issued upon acquisition (See Note 2), shares				2,514,468
Purchase of treasury stock, at cost		(3,044,000)			(3,044,000)
Purchase of treasury stock, at cost, shares					(1,038,609)
Amortization of share based compensation		9,493,000				9,493,000
Balance at Jun. 30, 2013		506,126,000		1,160,000	(34,772,000)	728,976,000	110,000	(189,348,000)	91,562,000
Common Stock, Shares, Outstanding at Jun. 30, 2013				117,861,088
Balance at Dec. 31, 2013		507,766,000		1,160,000	(48,084,000)	737,341,000	592,000	(183,243,000)	85,814,000
Common Stock, Shares, Outstanding at Dec. 31, 2013				115,026,633
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders		18,222,000						18,222,000
Net income (loss) attributable to redeemable non-controlling interests		9,403,000							9,403,000
Defined benefit plans	[1]	92,000
Foreign currency translation	[1]	(231,000)
Capital contributions									10,140,000
Capital withdrawals									(12,422,000)
Restricted stock awards issued, shares				4,028,356
Common stock issued upon acquisition (See Note 2)		0
Warrants issued		15,218,000
Stock options exercised (See Note 10)		116,000
Purchase of treasury stock, at cost		(16,648,000)		(16,648,000)
Purchase of treasury stock, at cost, shares				(4,088,186)
Amortization of share based compensation		10,477,000
Balance at Jun. 30, 2014		$ 535,012,000		$ 1,160,000	$ (64,732,000)	$ 763,152,000	$ 453,000	$ (165,021,000)	$ 92,935,000
Common Stock, Shares, Outstanding at Jun. 30, 2014				114,966,803

[1] During the periods presented, the Company did not have material reclassifications out of other comprehensive income.