Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings/(Accumulated deficit)	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Nonredeemable Noncontrolling Interests [Member]	Permanent Equity [Member]	Permanent Equity [Member] Cumulative Effect, Period of Adoption, Adjustment	Common Stock Class A	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	$ 809,855
Stockholders' Equity Attributable to Noncontrolling Interest								$ 94,320
Common stock											$ 334
Common stock, shares outstanding, start at Dec. 31, 2019											28,610,357
Balance, start at Dec. 31, 2019		$ (10)	$ (10)	$ 284,301	$ 1,110,635	$ (5)	$ (16,809)		$ 904,175	$ (10)			$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2019													120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2019	391,275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	(9,923)						(9,923)		(9,923)
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable								(1,198)	(1,198)
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	(60,990)
Foreign currency translation	0
Capital contributions	118,866							88,682	88,682
Capital withdrawals	(131,150)							(43,563)	(43,563)
Restricted stock awards issued, shares											591,763
Purchase of treasury stock, at cost, shares											(1,615,265)
Purchase of treasury stock, at cost	(20,891)			(20,891)					(20,891)
Common stock issued during period, shares, acquisitions											41,262
Common stock issued during period, value, acquisitions	618				618				618		$ 0
Preferred dividends paid	(1,698)								(1,698)					$ (1,698)
Dividends, Cash	(1,429)								(1,429)			$ (1,429)
Amortization of share based awards	8,281				8,281				8,281
Common stock, shares outstanding, end at Mar. 31, 2020											27,628,117
Balance, end at Mar. 31, 2020				305,192	1,119,534	(5)	(29,869)		923,044				$ 1
Preferred stock, shares outstanding, end at Mar. 31, 2020													120,750
Redeemable Non-controlling Interest, end at Mar. 31, 2020	318,001
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	784,803
Stockholders' Equity Attributable to Noncontrolling Interest								138,241
Common stock											$ 334
Total Stockholders' Equity	969,497
Stockholders' Equity Attributable to Noncontrolling Interest	199,624							199,624
Common stock											$ 334
Common stock, shares outstanding, start at Dec. 31, 2020											26,845,628
Balance, start at Dec. 31, 2020	1,169,121			346,870	1,130,138	(7)	185,901		1,169,121				$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2020													120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	147,504						147,504		147,504
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable								4,562	4,562
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	0
Foreign currency translation	4					4			4
Capital contributions	0							22,515	22,515
Capital withdrawals	0							(18,773)	(18,773)
Noncontrolling Interest, Decrease from Deconsolidation	0							74,813	74,813
Restricted stock awards issued, shares											705,512
Purchase of treasury stock, at cost, shares											(755,610)
Purchase of treasury stock, at cost	(26,904)			(26,904)					(26,904)
Common stock issued during period, shares, acquisitions											56,801
Common stock issued during period, value, acquisitions	2,202				2,202				2,202		$ 0
Preferred dividends paid	(1,698)								(1,698)					$ (1,698)
Dividends, Cash	(2,777)								(2,777)			$ (2,777)
Amortization of share based awards	19,037				19,037				19,037
Common stock, shares outstanding, end at Mar. 31, 2021											26,852,331
Balance, end at Mar. 31, 2021	1,239,980			$ 373,774	$ 1,151,377	$ (3)	$ 328,930		$ 1,239,980				$ 1
Preferred stock, shares outstanding, end at Mar. 31, 2021													120,750
Redeemable Non-controlling Interest, end at Mar. 31, 2021	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	1,106,865
Stockholders' Equity Attributable to Noncontrolling Interest	$ 133,115							$ 133,115
Common stock											$ 334