Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Preferred Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Retained Earnings/(Accumulated deficit) Cumulative Effect, Period of Adoption, Adjustment	Nonredeemable Noncontrolling Interests [Member]	Permanent Equity [Member]	Permanent Equity [Member] Cumulative Effect, Period of Adoption, Adjustment	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)		$ 1	$ (284,301)	$ 1,110,635	$ (5)	$ (16,809)	$ (10)		$ 904,175	$ (10)
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.	124,196							124,196			124,196
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable										13,081	13,081
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	(32,924)
Foreign currency translation	(2)						(2)				(2)
Capital contributions	184,223									163,019	163,019
Capital withdrawals	(181,863)									(48,995)	(48,995)
Increase in noncontrolling interest due to consolidation of entity	(48,596)									48,596	48,596
Noncontrolling Interest, Decrease from Deconsolidation	(360,711)
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		$ 0			926					926
Preferred dividends paid	(5,094)										(5,094)		$ (5,094)
Dividends, Cash	(4,144)							(4,144)			(4,144)
Amortization of share based awards	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	(334,135)	1,146,848	(7)	98,139			1,181,201
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]
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				$ 1	(314,971)	1,135,141	(5)	81,185			1,090,610
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
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable										9,232	9,232
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	0
Foreign currency translation	(2)						(2)				(2)
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 awards	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	(334,135)	1,146,848	(7)	98,139			1,181,201
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]
Total Stockholders' Equity	911,180
Stockholders' Equity Attributable to Noncontrolling Interest										270,021
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			$ 1	(346,870)	1,130,138	(7)	185,901			1,169,121
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.	230,630							230,630			230,630
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable										13,379	13,379
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	0
Foreign currency translation	5						5				5
Capital contributions	0									49,377	49,377
Capital withdrawals	0									(26,617)	(26,617)
Increase in noncontrolling interest due to consolidation of entity	0
Noncontrolling Interest, Decrease from Deconsolidation	0									74,813	74,813
Restricted stock awards issued, shares			1,879,397
Purchase of treasury stock, at cost, shares			(3,986,017)
Purchase of treasury stock, at cost	(149,052)				(149,052)						(149,052)
Debt Conversion, Converted Instrument, Shares Issued			2,938,841
Common stock issued during period, shares, acquisitions			56,801
Common stock issued during period, value, acquisitions	2,202		$ 0			2,202					2,202
Preferred dividends paid	(5,094)										(5,094)		(5,094)
Dividends, Cash	(9,388)							(9,388)			(9,388)
Amortization of share based awards	44,327					44,327					44,327
Common stock, shares outstanding, end at Sep. 30, 2021			27,734,650
Balance, end at Sep. 30, 2021	1,244,077			$ 1	(495,922)	1,176,667	(2)	402,049			1,244,077
Preferred stock, shares outstanding, end at Sep. 30, 2021				120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2021	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	1,097,344
Stockholders' Equity Attributable to Noncontrolling Interest										166,499
Common stock			$ 334
Common stock, shares outstanding, start at Jun. 30, 2021			28,995,801
Balance, start at Jun. 30, 2021				$ 1	(438,671)	1,166,165	(2)	369,517			1,263,843
Preferred stock, shares outstanding, start at Jun. 30, 2021				120,750
Redeemable Non-controlling Interest, start at Jun. 30, 2021	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	37,803							37,803			37,803
Net income (loss) attributable to non-controlling interest in consolidated subsidiaries and funds, Non-Reedemable										(4,938)	(4,938)
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	0
Foreign currency translation	0
Capital contributions	0									2,539	2,539
Capital withdrawals	0									(3,150)	(3,150)
Restricted stock awards issued, shares			327,697
Purchase of treasury stock, at cost, shares			(1,588,848)
Purchase of treasury stock, at cost	(57,251)				(57,251)						57,251
Preferred dividends paid	(1,698)										(1,698)		$ (1,698)
Dividends, Cash	(3,573)							(3,573)			(3,573)
Amortization of share based awards	10,502					10,502					10,502
Common stock, shares outstanding, end at Sep. 30, 2021			27,734,650
Balance, end at Sep. 30, 2021	1,244,077			$ 1	$ (495,922)	$ 1,176,667	$ (2)	$ 402,049			$ 1,244,077
Preferred stock, shares outstanding, end at Sep. 30, 2021				120,750
Redeemable Non-controlling Interest, end at Sep. 30, 2021	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total Stockholders' Equity	1,083,127
Stockholders' Equity Attributable to Noncontrolling Interest	$ 160,950									$ 160,950
Common stock			$ 334