Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Convertible Debt	Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock	Additional Paid-in Capital	Additional Paid-in Capital Convertible Debt	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Retained Earnings/(Accumulated deficit) Cumulative Effect, Period of Adoption, Adjustment	Nonredeemable Noncontrolling Interests [Member]	Permanent Equity [Member]	Permanent Equity [Member] Convertible Debt	Common Stock Class A	Common Stock Class A Cumulative Effect, Period of Adoption, Adjustment	Common Stock Class A Retained Earnings/(Accumulated deficit)	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity			$ (559)
Stockholders' Equity Attributable to Noncontrolling Interest										$ 105,587
Common stock													$ 324
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)	$ (559)		$ 853,606			$ (559)		$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2017																120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2017	335,017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	42,819							42,819			42,819
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable										12,308	12,308
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	24,752
Foreign currency translation	3						3				3
Capital contributions	107,117									7,888	7,888
Capital withdrawals	(249,963)									28,344	28,344
Noncontrolling Interest, Decrease from Deconsolidation										32,559	32,559
Restricted stock awards issued, shares													2,009,435
Purchase of treasury stock, at cost, shares													(3,203,595)
Purchase of treasury stock, at cost	(47,296)			(47,296)							(47,296)
Common stock issued during period, value, acquisitions	0
Embedded Derivative, No Longer Bifurcated, Amount Reclassified to Stockholders' Equity	(28,974)
Preferred dividends paid	(6,792)										(6,792)						$ (6,792)
Embedded Cash Conversion Option	21,195				21,195						21,195
Amortization of share based awards	37,018				37,018						37,018
Common stock, shares outstanding, end at Dec. 31, 2018													28,437,860
Balance, end at Dec. 31, 2018	794,407			(234,142)	1,062,877		(5)	(34,648)			859,287					$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2018																120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2018	216,923
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Noncontrolling Interest										64,880
Common stock													$ 324
Net income (loss) attributable to Cowen Inc.	24,631							24,631			24,631
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable										22,797	22,797
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	8,442
Foreign currency translation	0
Capital contributions	266,504									11,655	11,655
Capital withdrawals	(100,594)									5,012	5,012
Noncontrolling Interest, Decrease from Deconsolidation	0
Restricted stock awards issued, shares													2,407,857
Purchase of treasury stock, at cost, shares													(3,268,710)
Purchase of treasury stock, at cost	(50,159)			(50,159)							50,159
Common stock issued during period, shares, acquisitions													1,033,350
Common stock issued during period, value, acquisitions	14,446				14,436						14,446		$ 10
Preferred dividends paid	(6,792)										6,792						(6,792)
Embedded Cash Conversion Option	(596)				(596)						(596)
Amortization of share based awards	33,918				33,918						33,918
Common stock, shares outstanding, end at Dec. 31, 2019													28,610,357
Balance, end at Dec. 31, 2019	904,175			(284,301)	1,110,635		(5)	(16,809)	$ (10)		904,175			$ (10)		$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2019																120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2019	391,275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	809,855		$ (10)
Stockholders' Equity Attributable to Noncontrolling Interest	94,320									94,320
Common stock													$ 334
Net income (loss) attributable to Cowen Inc.	216,363							216,363			216,363
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable										23,625	23,625
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	(32,924)
Foreign currency translation	(2)						(2)				(2)
Capital contributions	184,223									201,223	201,223
Capital withdrawals	(181,863)									(78,251)	(78,251)
Increase in noncontrolling interest due to consolidation of entity	48,596									48,596	48,596
Noncontrolling Interest, Decrease from Deconsolidation	(360,711)									89,889	89,889
Restricted stock awards issued, shares													2,174,859
Purchase of treasury stock, at cost, shares													(4,014,282)
Purchase of treasury stock, at cost	(62,569)			(62,569)							62,569
Common stock issued during period, shares, acquisitions													74,694
Common stock issued during period, value, acquisitions	926				926						926		$ 0
Embedded Derivative, No Longer Bifurcated, Amount Reclassified to Stockholders' Equity		$ (29,645)				$ (29,645)						$ (29,645)
Preferred dividends paid	(6,792)										(6,792)						$ (6,792)
Dividends, Cash	(6,851)										(6,851)				$ (6,851)
Amortization of share based awards	48,222				48,222						48,222
Common stock, shares outstanding, end at Dec. 31, 2020													26,845,628
Balance, end at Dec. 31, 2020	1,169,121			$ (346,870)	$ 1,130,138		$ (7)	$ 185,901			$ 1,169,121					$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2020																120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	969,497
Stockholders' Equity Attributable to Noncontrolling Interest	$ 199,624									$ 199,624
Common stock													$ 334