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)	Call Option
Balance at Dec. 31, 2014	$ 677,675	$ (79,771)	$ 773,166	$ 17	$ (16,027)	$ 290	$ 0
Common Stock, Shares, Outstanding at Dec. 31, 2014						27,922,799.000
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 Inc.	43,729				43,729
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	15,246
Foreign currency translation	(17)			(17)
Capital contributions	110,178
Capital withdrawals	(24,589)
Common stock issuance upon acquisition, shares						137,156
Common stock issuance upon close of acquisition (see Note 2)	3,008		3,006			$ 2
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)
Preferred stock dividends (See Note 24)	(4,075)							$ (4,100)
Preferred stock issuance, net of issuance costs, shares							120,750
Preferred stock issuance, net of issuance costs	117,195		117,194				$ 1
Preferred Stock Dividends								(4,075)
Capped call option transaction			(15,878)						$ (15,878)
Stock Options exercised, shares						25,000
Stock options exercised, value	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
Redeemable Noncontrolling Interest at Dec. 31, 2015	186,911
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	(19,277)				(19,277)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	6,882
Foreign currency translation	(2)			(2)
Capital contributions	276,923
Capital withdrawals	(18,469)
Common stock issuance upon close of acquisition (see Note 2)	0
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)
Preferred stock dividends (See Note 24)	(6,792)							(6,800)
Preferred Stock Dividends								(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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	(60,882)				(60,882)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	23,791
Foreign currency translation	(6)			(6)
Capital contributions	119,112
Capital withdrawals	(81,504)
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						2,060,927
Purchase of treasury stock, at cost, shares						(2,322,474)
Purchase of treasury stock, at cost	(33,001)	(33,001)
Preferred stock dividends (See Note 24)	(6,792)							$ (6,792)
Amortization of share based compensation	28,443		28,443
Balance at Dec. 31, 2017	$ 748,019	$ (186,846)	$ 1,004,664	$ (8)	$ (70,116)	$ 324	$ 1
Common Stock, Shares, Outstanding at Dec. 31, 2017						29,632,020
Preferred stock, shares outstanding at Dec. 31, 2017	120,750						120,750
Redeemable Noncontrolling Interest at Dec. 31, 2017	$ 440,604