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)
Total Stockholders' Equity	$ 748,019
Common stock									$ 324
Common stock, shares outstanding, start at Dec. 31, 2017									29,632,020
Balance, start at Dec. 31, 2017		$ (186,846)	$ 1,004,664	$ (8)	$ (70,116)				$ 853,607	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2017										120,750
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax	(559)						$ (559)
Net income (loss) attributable to Cowen Inc.	37,740				37,740		37,740
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						$ 15,297	15,297
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	22,501
Foreign currency translation	5			5
Capital contributions	17,648					7,887	7,887
Capital withdrawals	(243,807)					27,254	27,254
Deconsolidation of investment funds	(32,559)					32,559	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)					28,477
Preferred stock dividends	(5,094)						5,094				$ (5,094)
Embedded Cash Conversion Option	28,974		28,974				28,974
Amortization of share based compensation	27,636		27,636				27,636
Common stock, shares outstanding, end at Sep. 30, 2018									29,142,547
Balance, end at Sep. 30, 2018		(215,323)	1,061,274	(3)	(38,029)				$ 877,203	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2018										120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2018	131,358							$ 131,358
Total Stockholders' Equity	793,207
Common stock									$ 324
Stockholders' Equity Attributable to Noncontrolling Interest						67,407
Common stock, shares outstanding, start at Jun. 30, 2018									29,609,472
Balance, start at Jun. 30, 2018		(208,009)	1,052,717	(7)	(51,819)				$ 860,614	$ 1
Preferred stock, shares outstanding, start at Jun. 30, 2018										121,000
Redeemable Non-controlling Interest, start at Jun. 30, 2018	342,944
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	15,488				15,488		15,488
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						2,622	2,622
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds								1,487
Foreign currency translation	4			4			4
Capital contributions	5,659					382	382
Capital withdrawals	(218,732)					1,452	1,452
Restricted stock awards issued, shares									30,228
Purchase of treasury stock, at cost, shares									(497,153)
Purchase of treasury stock, at cost	(7,314)	(7,314)					7,314
Preferred stock dividends	(1,698)						1,698				(1,698)
Embedded Cash Conversion Option	1		1				1
Amortization of share based compensation	8,556		8,556				8,556
Common stock, shares outstanding, end at Sep. 30, 2018									29,142,547
Balance, end at Sep. 30, 2018		(215,323)	1,061,274	(3)	(38,029)				$ 877,203	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2018										120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2018	131,358							131,358
Total Stockholders' Equity	808,244
Common stock									324
Stockholders' Equity Attributable to Noncontrolling Interest						68,959
Total Stockholders' Equity	794,407
Common stock									$ 324
Stockholders' Equity Attributable to Noncontrolling Interest	64,880					64,880
Common stock, shares outstanding, start at Dec. 31, 2018									28,437,860
Balance, start at Dec. 31, 2018	$ 859,287	(234,142)	1,062,877	(5)	(34,648)				$ 859,287	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2018	120,750									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.	$ 19,408				19,408		19,408
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						11,375	11,375
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	(4,187)
Foreign currency translation	0			0
Capital contributions	213,391					11,442	11,442
Capital withdrawals	(77,703)					2,938	2,938
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									1,668,032
Purchase of treasury stock, at cost, shares									(2,126,105)
Purchase of treasury stock, at cost	(32,740)	(32,740)					32,740
Preferred stock dividends	(5,094)						5,094				(5,094)
Embedded Cash Conversion Option	(596)		(596)				(596)
Amortization of share based compensation	27,224		27,224				27,224
Common stock, shares outstanding, end at Sep. 30, 2019									29,013,137
Balance, end at Sep. 30, 2019	$ 901,814	(266,882)	1,103,941	(5)	(20,334)				$ 901,814	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2019	120,750									120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2019								348,424
Total Stockholders' Equity	$ 817,424
Common stock									$ 334
Stockholders' Equity Attributable to Noncontrolling Interest						75,923
Common stock, shares outstanding, start at Jun. 30, 2019									29,480,287
Balance, start at Jun. 30, 2019		(254,357)	1,093,898	(3)	(22,449)				$ 893,347	$ 1
Preferred stock, shares outstanding, start at Jun. 30, 2019										120,750
Redeemable Non-controlling Interest, start at Jun. 30, 2019								359,643
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	3,813				3,813		3,813
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						9,618	9,618
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	(6,848)
Foreign currency translation	(2)			(2)			(2)
Capital contributions	21,905					332	332
Capital withdrawals	(26,276)					1,114	1,114
Restricted stock awards issued, shares									331,213
Purchase of treasury stock, at cost, shares									(798,363)
Purchase of treasury stock, at cost	(12,525)	(12,525)					12,525
Preferred stock dividends	(1,698)						1,698				$ (1,698)
Amortization of share based compensation	10,043		10,043				$ 10,043
Common stock, shares outstanding, end at Sep. 30, 2019									29,013,137
Balance, end at Sep. 30, 2019	$ 901,814	$ (266,882)	$ 1,103,941	$ (5)	$ (20,334)				$ 901,814	$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2019	120,750									120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2019								$ 348,424
Total Stockholders' Equity	$ 817,055
Common stock									$ 334
Stockholders' Equity Attributable to Noncontrolling Interest	$ 84,759					$ 84,759