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)	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.	13,068				13,068
Net income (loss) attributable to redeemable non-controlling interests	10,980
Foreign currency translation	(4)			(4)
Capital contributions	88,740
Capital withdrawals	(14,551)
Restricted stock awards issued, shares								4,027,405
Purchase of treasury stock, at cost, shares								(9,269,163)
Purchase of treasury stock, at cost	(50,253)
Preferred stock issuance, net of issuance costs (See Note 14) (in shares)						120,750
Preferred stock issuance, net of issuance costs (See Note 14)	117,195		117,194			$ 1
Dividends							$ (2,358)
Preferred stock dividends	(2,358)
Common stock issued upon acquisition, shares								548,625
Common stock issuance upon close of acquisition (see Note 2)	3,008		3,002					$ 6
Capped call option transaction									$ (15,878)
Income tax effect from share based compensation	3,714
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	16,211
Balance at Sep. 30, 2015	762,773	(130,024)	896,933	13	(5,317)	$ 1		$ 1,167
Common Stock, Shares, Outstanding at Sep. 30, 2015								107,098,068
Preferred stock, shares outstanding at Sep. 30, 2015						120,750
Redeemable Noncontrolling Interest at Sep. 30, 2015	171,245
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.	(17,373)				(17,373)
Net income (loss) attributable to redeemable non-controlling interests	(2,524)
Foreign currency translation	(3)			(3)
Capital contributions	236,720
Capital withdrawals	(16,182)
Deconsolidation of entity	(73,042)
Restricted stock awards issued, shares								6,011,437
Purchase of treasury stock, at cost, shares								(4,301,938)
Purchase of treasury stock, at cost	(14,807)
Dividends							$ (5,094)
Preferred stock dividends	(5,094)
Common stock issuance upon close of acquisition (see Note 2)	0
Income tax effect from share based compensation	(763)
Amortization of share based compensation	19,271
Balance at Sep. 30, 2016	$ 771,224	$ (152,163)	$ 921,062	$ (3)	$ 1,160	$ 1		$ 1,167
Common Stock, Shares, Outstanding at Sep. 30, 2016								107,314,157
Preferred stock, shares outstanding at Sep. 30, 2016	120,750					120,750
Redeemable Noncontrolling Interest at Sep. 30, 2016	$ 331,883