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]	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.	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 non-controlling interests in consolidated subsidiaries and investment funds	(4,187)
Capital contributions	213,391					11,442	11,442
Capital withdrawals	(77,703)					(2,938)	(2,938)
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)
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	(5,094)						(5,094)				$ (5,094)
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		(266,882)	1,103,941	(5)	(20,334)			$ 901,814		$ 1
Preferred stock, shares outstanding, end at Sep. 30, 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 Sep. 30, 2019	348,424
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation	0
Noncontrolling Interest, Decrease from Deconsolidation	0
Total Stockholders' Equity	817,424
Stockholders' Equity Attributable to Noncontrolling Interest						75,923
Common stock								$ 334
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 non-controlling interests in consolidated subsidiaries and investment funds	(6,848)
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 dividends paid	(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		(266,882)	1,103,941	(5)	(20,334)			$ 901,814		$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2019										120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2019	348,424
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation	(2)			(2)			(2)
Total Stockholders' Equity	817,055
Stockholders' Equity Attributable to Noncontrolling Interest						84,759
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
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.	124,196				124,196		124,196
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						13,081	13,081
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	(32,924)
Capital contributions	184,223					163,019	163,019
Capital withdrawals	(181,863)					(48,995)	(48,995)
Restricted stock awards issued, shares								1,371,723
Purchase of treasury stock, at cost, shares								(3,487,439)
Purchase of treasury stock, at cost	(49,834)	(49,834)					49,834
Common stock issued during period, shares, acquisitions								74,694
Common stock issued during period, value, acquisitions	926		926				926	$ 0
Preferred dividends paid	(5,094)						(5,094)				(5,094)
Dividends, Cash	(4,144)						(4,144)		$ (4,144)
Amortization of share based compensation	35,287		35,287				35,287
Common stock, shares outstanding, end at Sep. 30, 2020								26,569,335
Balance, end at Sep. 30, 2020	1,181,201	(334,135)	1,146,848	(7)	98,139			$ 1,181,201		$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2020										120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation	(2)			(2)			(2)
Increase in noncontrolling interest due to consolidation of entity	48,596					(48,596)	(48,596)
Noncontrolling Interest, Decrease from Deconsolidation	(360,711)
Total Stockholders' Equity	901,685
Stockholders' Equity Attributable to Noncontrolling Interest						188,925
Common stock								$ 334
Common stock, shares outstanding, start at Jun. 30, 2020								27,641,600
Balance, start at Jun. 30, 2020		(314,971)	1,135,141	(5)	81,185			$ 1,090,610		$ 1
Preferred stock, shares outstanding, start at Jun. 30, 2020										120,750
Redeemable Non-controlling Interest, start at Jun. 30, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	20,284				20,284		20,284
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable						9,232	9,232
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	0
Capital contributions	0					74,337	74,337
Capital withdrawals	0					(2,473)	(2,473)
Restricted stock awards issued, shares								88,811
Purchase of treasury stock, at cost, shares								(1,161,076)
Purchase of treasury stock, at cost	(19,164)	(19,164)					19,164
Preferred dividends paid	(1,698)						(1,698)				$ (1,698)
Dividends, Cash	(1,632)						(1,632)		$ (1,632)
Amortization of share based compensation	11,707		11,707				11,707
Common stock, shares outstanding, end at Sep. 30, 2020								26,569,335
Balance, end at Sep. 30, 2020	1,181,201	$ (334,135)	$ 1,146,848	(7)	$ 98,139			$ 1,181,201		$ 1
Preferred stock, shares outstanding, end at Sep. 30, 2020										120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation	(2)			$ (2)			$ (2)
Total Stockholders' Equity	911,180
Stockholders' Equity Attributable to Noncontrolling Interest	$ 270,021					$ 270,021
Common stock								$ 334