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)	Call Option Additional Paid-in Capital	Cumulative Preferred Stock	Cumulative Preferred Stock Retained Earnings/(Accumulated deficit)	Common Stock Class A
Balance at Dec. 31, 2013		$ 507,766		$ (48,084)	$ 737,341	$ 592	$ (183,243)		$ 0		$ 1,160
Common Stock, Shares, Outstanding at Dec. 31, 2013											115,026,633
Preferred stock, shares outstanding at Dec. 31, 2013									0
Redeemable Noncontrolling Interest at Dec. 31, 2013		85,814
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc.		24,745					24,745
Net income (loss) attributable to redeemable non-controlling interests		13,409
Defined benefit plans		(87)	[1]			(87)
Foreign currency translation		(233)	[1]			(233)
Capital contributions		10,273
Capital withdrawals		(18,067)
Restricted stock awards issued, shares											4,242,989
Purchase of treasury stock, at cost, shares											(5,634,819)
Purchase of treasury stock, at cost		(22,937)
Preferred stock dividends		0
Common stock issuance upon acquisition		0
Warrants issued		15,218
Stock options exercised, shares											0
Stock Options Exercised		116			116
Amortization of share based compensation		14,286			14,286
Balance at Sep. 30, 2014		538,874		(71,021)	766,961	272	(158,498)		$ 0		$ 1,160
Common Stock, Shares, Outstanding at Sep. 30, 2014											113,634,803
Preferred stock, shares outstanding at Sep. 30, 2014									0
Redeemable Noncontrolling Interest at Sep. 30, 2014		91,429
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
Preferred stock, shares outstanding at Dec. 31, 2014		0							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
Defined benefit plans	[1]	0
Foreign currency translation		(4)	[1]			(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									120,750
Preferred stock issuance, net of issuance costs		117,195			117,194				$ 1
Preferred stock dividends		(2,358)								$ (2,358)
Common stock issuance upon acquisition, shares											548,625
Common stock issuance upon acquisition		3,008			3,002						$ 6
Capped call option transaction		(15,878)						$ (15,878)
Income tax effect from share based compensation		3,714			3,714
Stock options exercised, shares											100,002
Stock Options Exercised		395			394						$ 1
Amortization of share based compensation		16,211			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							120,750
Redeemable Noncontrolling Interest at Sep. 30, 2015		$ 171,245

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