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)	Nonredeemable Noncontrolling Interests [Member]	Permanent Equity [Member]	Redeemable Noncontrolling Interests [Member]	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	$ 794,407
Stockholders' Equity Attributable to Noncontrolling Interest						$ 64,880
Common stock									$ 324
Common stock, shares outstanding, start at Dec. 31, 2018									28,437,860
Balance, start at Dec. 31, 2018		$ (234,142)	$ 1,062,877	$ (5)	$ (34,648)				$ 859,287		$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2018											120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2018								$ 216,923
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	9,783				9,783		$ 9,783
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						(288)	(288)
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	800
Capital contributions	159,948					2,025	2,025
Capital withdrawals	(29,157)					(1,174)	(1,174)
Restricted stock awards issued, shares									751,436
Purchase of treasury stock, at cost, shares									(641,959)
Purchase of treasury stock, at cost	(9,377)	(9,377)					(9,377)
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 stock dividends	(1,698)						(1,698)					$ (1,698)
Amortization of share based compensation	7,817		7,817				7,817
Common stock, shares outstanding, end at Mar. 31, 2019									29,580,687
Balance, end at Mar. 31, 2019		(243,519)	1,084,534	(5)	(26,563)				$ 880,225		$ 1
Preferred stock, shares outstanding, end at Mar. 31, 2019											120,750
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			(596)				(596)
Redeemable Non-controlling Interest, end at Mar. 31, 2019								348,514
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	814,782
Stockholders' Equity Attributable to Noncontrolling Interest						65,443
Common stock									334
Total Stockholders' Equity	809,855
Stockholders' Equity Attributable to Noncontrolling Interest	94,320					94,320
Common stock									$ 334
Common stock, shares outstanding, start at Dec. 31, 2019									28,610,357
Balance, start at Dec. 31, 2019	$ 904,175	(284,301)	1,110,635	(5)	(16,809)				$ 904,175		$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2019	120,750										120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2019								391,275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ (10)				(10)		(10)
Net income (loss) attributable to Cowen Inc.	(9,923)				(9,923)		(9,923)
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						(1,198)	(1,198)
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	(60,990)
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 stock dividends	(1,698)						(1,698)					$ (1,698)
Dividends, Cash	1,429						1,429			$ 1,429
Amortization of share based compensation	8,281		8,281				$ 8,281
Common stock, shares outstanding, end at Mar. 31, 2020									27,628,117
Balance, end at Mar. 31, 2020	$ 923,044	$ (305,192)	$ 1,119,534	$ (5)	$ (29,869)				$ 923,044		$ 1
Preferred stock, shares outstanding, end at Mar. 31, 2020	120,750										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					$ 138,241
Common stock									$ 334