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)	Common Stock Class A	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Balance at Dec. 31, 2015	$ 789,993	$ (137,356)	$ 903,429	$ 0	$ 23,627	$ 292	$ 1
Common Stock, Shares, Outstanding at Dec. 31, 2015						26,401,163
Preferred stock, shares outstanding at Dec. 31, 2015							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 in consolidated subsidiaries and funds	(2,524)
Foreign currency translation	(3)			(3)
Capital contributions	236,720
Capital withdrawals	(16,182)
Common stock issuance upon close of acquisition (see Note 2)	0
Deconsolidation of funds	(73,042)
Restricted stock awards issued, shares						1,502,859
Purchase of treasury stock, at cost, shares						(1,075,484)
Purchase of treasury stock, at cost	(14,807)	(14,807)
Preferred stock dividends (See Note 18)	(5,094)							$ (5,094)
Income tax effect from share based compensation	(763)		(763)
Amortization of share based compensation	19,271		19,271
Balance at Sep. 30, 2016	771,224	(152,163)	921,937	(3)	1,160	$ 292	$ 1
Common Stock, Shares, Outstanding at Sep. 30, 2016						26,828,538
Preferred stock, shares outstanding at Sep. 30, 2016							120,750
Redeemable Noncontrolling Interest at Sep. 30, 2016	331,883
Balance at Dec. 31, 2016	$ 772,650	(153,845)	928,646	(2)	(2,442)	$ 292	$ 1
Common Stock, Shares, Outstanding at Dec. 31, 2016						26,731,289
Preferred stock, shares outstanding at Dec. 31, 2016	120,750						120,750
Redeemable Noncontrolling Interest at Dec. 31, 2016	$ 379,205
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc.	15,090				15,090
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	35,412
Foreign currency translation	(6)			(6)
Capital contributions	60,322
Capital withdrawals	(53,278)
Common stock issuance upon acquisition, shares						3,162,278
Common stock issuance upon close of acquisition (see Note 2)	47,607		47,575			$ 32
Restricted stock awards issued, shares						1,985,618
Purchase of treasury stock, at cost, shares						(791,727)
Purchase of treasury stock, at cost	(11,433)	(11,433)
Preferred stock dividends (See Note 18)	(5,094)							$ (5,094)
Amortization of share based compensation	24,702		24,702
Balance at Sep. 30, 2017	$ 843,516	$ (165,278)	$ 1,000,923	$ (8)	$ 7,554	$ 324	$ 1
Common Stock, Shares, Outstanding at Sep. 30, 2017						31,087,458
Preferred stock, shares outstanding at Sep. 30, 2017	120,750						120,750
Redeemable Noncontrolling Interest at Sep. 30, 2017	$ 421,661