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	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Common stock									$ 292
Stockholders' Equity Attributable to Noncontrolling Interest						$ 98,678
Common stock, shares outstanding, start at Dec. 31, 2016									26,731,289
Balance, start at Dec. 31, 2016	$ 772,650	$ (153,845)	$ 928,646	$ (2)	$ (2,442)				$ 871,328	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2016										120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2016	280,527
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	(60,882)				(60,882)		$ (60,882)
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						16,134	16,134
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	7,657
Foreign currency translation	(6)			(6)			(6)
Capital contributions	89,034					30,078	30,078
Capital withdrawals	(42,201)					39,303	39,303
Common stock issued during period, shares, acquisitions									3,162,278
Common stock issued during period, value, acquisitions	47,607		47,575				47,607		$ 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)					(33,001)
Preferred stock dividends	(6,792)						(6,792)				$ (6,792)
Amortization of share based compensation	28,443		28,443				28,443
Common stock, shares outstanding, end at Dec. 31, 2017									29,632,020
Balance, end at Dec. 31, 2017	748,019	(186,846)	1,004,664	(8)	(70,116)				$ 853,606	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2017										120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2017	335,017
Common stock									$ 324
Stockholders' Equity Attributable to Noncontrolling Interest						105,587
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax	(559)						(559)
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
Deconsolidation of investment funds	(32,559)					(32,559)	(32,559)
Common stock issued during period, value, acquisitions	0
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
Preferred stock dividends	(6,792)						6,792				(6,792)
Embedded Cash Conversion Option	28,974		21,195				21,195
Amortization of share based compensation	37,018		37,018				37,018
Common stock, shares outstanding, end at Dec. 31, 2018									28,437,860
Balance, end at Dec. 31, 2018	$ 859,287	(234,142)	1,062,877	(5)	(34,648)				$ 859,287	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2018	120,750									120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2018								$ 216,923
Total Stockholders' Equity	$ 794,407
Common stock									$ 324
Stockholders' Equity Attributable to Noncontrolling Interest	64,880					64,880
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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			0			0
Capital contributions	266,504					11,655	11,655
Capital withdrawals	(100,594)					5,012	5,012
Deconsolidation of investment funds	0
Common stock issued during period, shares, acquisitions									1,033,350
Common stock issued during period, value, acquisitions	14,446		14,436				14,446		$ 10
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
Preferred stock dividends	(6,792)						6,792				$ (6,792)
Embedded Cash Conversion Option	(596)		(596)				(596)
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net	21,195
Amortization of share based compensation	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)				$ 904,175	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2019	120,750									120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2019								$ 391,275
Total Stockholders' Equity	$ 809,855
Common stock									$ 334
Stockholders' Equity Attributable to Noncontrolling Interest	$ 94,320					$ 94,320