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)
Common stock, shares outstanding, start at Dec. 31, 2016						26,731,289.000
Balance, start at Dec. 31, 2016	$ 772,650	$ (153,845)	$ 928,646	$ (2)	$ (2,442)	$ 292	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2016							120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2016	379,205
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen 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)
Deconsolidation of funds	$ 0
Common stock issued during period, shares, acquisitions	3,162,278
Common stock issued during period, value, acquisitions	$ 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	(5,094)							$ (5,094)
Embedded Cash Conversion Option	0
Amortization of share based compensation	24,702		24,702
Common stock, shares outstanding, end at Sep. 30, 2017						31,087,458
Balance, end at Sep. 30, 2017	843,516	(165,278)	1,000,923	(8)	7,554	$ 324	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2017							120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2017	421,661
Common stock, shares outstanding, start at Dec. 31, 2017						29,632,020
Balance, start at Dec. 31, 2017	$ 748,019	(186,846)	1,004,664	(8)	(70,116)	$ 324	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2017	120,750						120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2017	$ 440,604
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax	(559)
Net income (loss) attributable to Cowen Inc.	37,740				37,740
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	41,053
Foreign currency translation	5			5
Capital contributions	25,175
Capital withdrawals	(271,061)
Deconsolidation of funds	(32,559)
Common stock issued during period, value, acquisitions	0
Restricted stock awards issued, shares						1,469,983
Purchase of treasury stock, at cost, shares						(1,959,456)
Purchase of treasury stock, at cost	(28,477)	(28,477)
Preferred stock dividends	(5,094)							$ (5,094)
Embedded Cash Conversion Option	28,974		28,974
Amortization of share based compensation	27,636		27,636
Common stock, shares outstanding, end at Sep. 30, 2018						29,142,547
Balance, end at Sep. 30, 2018	$ 808,244	$ (215,323)	$ 1,061,274	$ (3)	$ (38,029)	$ 324	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2018	120,750						120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2018	$ 203,212