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	Preferred Stock Additional Paid-in Capital	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)	Options
Balance at Dec. 31, 2013	$ 507,766	$ (48,084)	$ 738,211	$ 592	$ (183,243)	$ 290	$ 0
Common Stock, Shares, Outstanding at Dec. 31, 2013						28,756,658
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.	167,216				167,216
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	15,564
Defined benefit plans	344			344
Foreign currency translation	(231)			(231)
Capital contributions	10,441
Capital withdrawals	(24,585)
Deconsolidation of funds	(1,158)
Restricted stock awards issued, shares						1,066,334
Purchase of treasury stock, at cost, shares						(1,908,526)
Purchase of treasury stock, at cost	(31,687)	(31,687)
Common stock issuance upon close of acquisition (see Note 2)	0
Preferred stock dividends (See Note 20)	0
Stock Options exercised, shares						8,333
Stock options exercised	116		116
Income tax effect from share based compensation	1,324		1,324
Warrants issued	15,218		15,218
Amortization of share based compensation	18,297		18,297
Balance at Dec. 31, 2014	677,675	(79,771)	773,166	17	(16,027)	$ 290	$ 0
Common Stock, Shares, Outstanding at Dec. 31, 2014						27,922,799
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.	43,729				43,729
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	15,246
Defined benefit plans	0
Foreign currency translation	(17)			(17)
Capital contributions	110,178
Capital withdrawals	(24,589)
Restricted stock awards issued, shares						1,068,227
Purchase of treasury stock, at cost, shares						(2,752,019)
Purchase of treasury stock, at cost	(57,585)	(57,585)
Common stock issued upon acquisition, shares						137,156
Common stock issuance upon close of acquisition (see Note 2)	3,008		3,006			$ 2
Preferred stock issuance, net of issuance costs (See Note 20) (in shares)							120,750
Preferred stock issuance, net of issuance costs (See Note 14)	117,195						$ 1	$ 117,194
Preferred stock dividends (See Note 20)									$ (4,075)
Preferred stock dividends (See Note 20)	(4,075)
Capped call option transaction (See Note 20)			(15,878)							$ (15,878)
Stock Options exercised, shares						25,000
Stock options exercised	395		395
Income tax effect from share based compensation	3,806		3,806
Amortization of share based compensation	21,740		21,740
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						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.	(19,277)				(19,277)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	6,882
Defined benefit plans	0
Foreign currency translation	(2)			(2)
Capital contributions	276,923
Capital withdrawals	(18,469)
Deconsolidation of funds	(73,042)
Restricted stock awards issued, shares						1,511,995
Purchase of treasury stock, at cost, shares						(1,181,869)
Purchase of treasury stock, at cost	(16,489)	(16,489)
Common stock issuance upon close of acquisition (see Note 2)	0
Preferred stock dividends (See Note 20)									$ (6,792)
Preferred stock dividends (See Note 20)	(6,792)
Income tax effect from share based compensation	(822)		(822)
Amortization of share based compensation	26,039		26,039
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