Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Cumulative Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)	Common Stock Class A	Options
Balance at Dec. 31, 2014	$ 677,675	$ (79,771)	$ 772,296	$ 17	$ (16,027)	$ 0		$ 1,160
Common Stock, Shares, Outstanding at Dec. 31, 2014								111,691,199.000
Preferred stock, shares outstanding at Dec. 31, 2014						0
Redeemable Noncontrolling Interest at Dec. 31, 2014	86,076
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc.	23,414				23,414
Net income (loss) attributable to redeemable non-controlling interests	6,636
Foreign currency translation	(1)			(1)
Capital contributions	76,846
Capital withdrawals	(7,476)
Restricted stock awards issued, shares								3,906,942
Purchase of treasury stock, at cost, shares								(5,704,881)
Purchase of treasury stock, at cost	(31,089)
Preferred stock issuance, net of issuance costs (See Note 14) (in shares)						120,750
Preferred stock issuance, net of issuance costs (See Note 14)	117,310		117,309			$ 1
Dividends							$ (755)
Preferred stock dividends	(755)
Capped call option transaction									$ (15,878)
Income tax effect from share based compensation	3,695
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period								100,002
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures								$ 1
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options	395		394
Amortization of share based compensation	10,870
Balance at Jun. 30, 2015	785,636	(110,860)	888,686	16	6,632	$ 1		$ 1,161
Common Stock, Shares, Outstanding at Jun. 30, 2015								109,993,262
Preferred stock, shares outstanding at Jun. 30, 2015						120,750
Redeemable Noncontrolling Interest at Jun. 30, 2015	162,082
Balance at Dec. 31, 2015	$ 789,993	(137,356)	902,554	0	23,627	$ 1		$ 1,167
Common Stock, Shares, Outstanding at Dec. 31, 2015								105,604,658
Preferred stock, shares outstanding at Dec. 31, 2015	120,750					120,750
Redeemable Noncontrolling Interest at Dec. 31, 2015	$ 186,911
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc.	(14,154)				(14,154)
Net income (loss) attributable to redeemable non-controlling interests	(21,002)
Foreign currency translation	(5)			(5)
Capital contributions	231,871
Capital withdrawals	(9,615)
Deconsolidation of entity	(73,042)
Restricted stock awards issued, shares								5,836,014
Purchase of treasury stock, at cost, shares								(4,234,046)
Purchase of treasury stock, at cost	(14,576)
Dividends							$ (3,396)
Preferred stock dividends	(3,396)
Income tax effect from share based compensation	(744)
Amortization of share based compensation	13,036
Balance at Jun. 30, 2016	$ 770,154	$ (151,932)	$ 914,846	$ (5)	$ 6,077	$ 1		$ 1,167
Common Stock, Shares, Outstanding at Jun. 30, 2016								107,206,626
Preferred stock, shares outstanding at Jun. 30, 2016	120,750					120,750
Redeemable Noncontrolling Interest at Jun. 30, 2016	$ 315,123