Condensed Consolidated Statements of Changes in Equity (USD $)	Total USD ($)		Common Stock USD ($)	Treasury Stock USD ($)	Additional Paid-in Capital USD ($)	Accumulated Other Comprehensive Income (Loss) USD ($)	Retained Earnings/(Accumulated deficit) USD ($)	Redeemable Non-controlling Interest USD ($)	Common Stock Class A
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	2,128,000						2,128,000
Net income (loss) attributable to redeemable non-controlling interests	10,509,000							10,509,000
Defined benefit plans	(207,000)	[1]				(207,000)
Foreign currency translation	(10,000)	[1]				(10,000)
Capital contributions								11,656,000
Capital withdrawals								(17,689,000)
Restricted stock awards issued, shares			4,288,108
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	(5,879,000)			(5,879,000)
Treasury stock reissued, shares				24,744
Treasury stock reissued, cost				90,000
Treasury stock reissued during the period net of gain(loss)	65,000
Treasury Stock Reissued at Lower than Repurchase Price							(25,000)
Purchase of treasury stock, at cost, shares				(1,863,684)
Amortization of share based compensation	13,803,000				13,803,000
Balance at Sep. 30, 2013	511,306,000		1,160,000	(37,517,000)	733,286,000	139,000	(185,762,000)	90,179,000
Common Stock, Shares, Outstanding at Sep. 30, 2013			117,411,528
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						115,026,633
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders	24,745,000						24,745,000
Net income (loss) attributable to redeemable non-controlling interests	13,409,000							13,409,000
Defined benefit plans	(87,000)	[1]				(87,000)
Foreign currency translation	(233,000)	[1]				(233,000)
Capital contributions								10,273,000
Capital withdrawals								(18,067,000)
Restricted stock awards issued, shares			4,242,989
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	(22,937,000)			(22,937,000)
Treasury stock reissued during the period net of gain(loss)	0
Purchase of treasury stock, at cost, shares				(5,634,819)
Amortization of share based compensation	14,286,000
Balance at Sep. 30, 2014	$ 538,874,000		$ 1,160,000	$ (71,021,000)	$ 766,961,000	$ 272,000	$ (158,498,000)	$ 91,429,000
Common Stock, Shares, Outstanding at Sep. 30, 2014			113,634,803						113,634,803

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