Consolidated Statements of Changes in Equity - USD ($)	Total	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Common Stock Class A	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Common stock, shares outstanding, start at Dec. 31, 2017						29,632,020.00
Balance, start at Dec. 31, 2017	$ 748,019,000	$ (186,846,000)	$ 1,004,664,000	$ (8,000)	$ (70,116,000)	$ 324,000	$ 1,000
Preferred stock, shares outstanding, start at Dec. 31, 2017							120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2017	440,604,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax	(559,000)
Net income (loss) attributable to Cowen Inc.	22,252,000				22,252,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	35,763,000
Foreign currency translation	1,000			1,000
Capital contributions	19,136,000
Capital withdrawals	(50,877,000)
Deconsolidation of investment funds	(32,559,000)
Common stock issued during period, value, acquisitions	0
Restricted stock awards issued, shares						1,439,755
Purchase of treasury stock, at cost, shares						(1,462,303)
Purchase of treasury stock, at cost	(21,163,000)	(21,163,000)
Preferred stock dividends	(3,396,000)							$ (3,396,000)
Embedded Cash Conversion Option	28,973,000		28,973,000
Amortization of share based compensation	19,080,000		19,080,000
Common stock, shares outstanding, end at Jun. 30, 2018						29,609,472
Balance, end at Jun. 30, 2018	793,207,000	(208,009,000)	1,052,717,000	(7,000)	(51,819,000)	$ 324,000	$ 1,000
Preferred stock, shares outstanding, end at Jun. 30, 2018							120,750
Redeemable Non-controlling Interest, end at Jun. 30, 2018	412,067,000
Common stock, shares outstanding, start at Mar. 31, 2018						29,517,000
Balance, start at Mar. 31, 2018	758,787,000	(200,915,000)	1,014,904,000	(7,000)	(55,520,000)	$ 324,000	$ 1,000
Preferred stock, shares outstanding, start at Mar. 31, 2018							121,000
Redeemable Non-controlling Interest, start at Mar. 31, 2018	392,326,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	5,399,000				5,399,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	24,607,000
Foreign currency translation	0
Capital contributions	10,876,000
Capital withdrawals	(15,742,000)
Restricted stock awards issued, shares						559,000
Purchase of treasury stock, at cost, shares						(466,000)
Purchase of treasury stock, at cost	(7,094,000)	(7,094,000)
Preferred stock dividends	(1,698,000)							(1,698,000)
Embedded Cash Conversion Option	28,973,000		28,973,000
Amortization of share based compensation	8,840,000		8,840,000
Common stock, shares outstanding, end at Jun. 30, 2018						29,609,472
Balance, end at Jun. 30, 2018	793,207,000	(208,009,000)	1,052,717,000	(7,000)	(51,819,000)	$ 324,000	$ 1,000
Preferred stock, shares outstanding, end at Jun. 30, 2018							120,750
Redeemable Non-controlling Interest, end at Jun. 30, 2018	412,067,000
Common stock, shares outstanding, start at Dec. 31, 2018						28,437,860
Balance, start at Dec. 31, 2018	$ 794,407,000	(234,142,000)	1,062,877,000	(5,000)	(34,648,000)	$ 324,000	$ 1,000
Preferred stock, shares outstanding, start at Dec. 31, 2018	120,750						120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2018	$ 284,780,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	15,595,000				15,595,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	6,537,000
Foreign currency translation	2,000			2,000
Capital contributions	202,595,000
Capital withdrawals	(53,600,000)
Common stock issued during period, shares, acquisitions						1,033,350
Common stock issued during period, value, acquisitions	14,446,000		14,436,000			$ 10,000
Restricted stock awards issued, shares						1,336,819
Purchase of treasury stock, at cost, shares						(1,327,742)
Purchase of treasury stock, at cost	(20,215,000)	(20,215,000)
Preferred stock dividends	(3,396,000)							(3,396,000)
Embedded Cash Conversion Option			(596,000)
Amortization of share based compensation	17,181,000		17,181,000
Common stock, shares outstanding, end at Jun. 30, 2019						29,480,287.000
Balance, end at Jun. 30, 2019	$ 817,424,000	(254,357,000)	1,093,898,000	(3,000)	(22,449,000)	$ 334,000	$ 1,000
Preferred stock, shares outstanding, end at Jun. 30, 2019	120,750						120,750
Redeemable Non-controlling Interest, end at Jun. 30, 2019	$ 440,312,000
Common stock, shares outstanding, start at Mar. 31, 2019						29,580,687
Balance, start at Mar. 31, 2019	814,782,000	(243,519,000)	1,084,534,000	(5,000)	(26,563,000)	$ 334,000	$ 1,000
Preferred stock, shares outstanding, start at Mar. 31, 2019							120,750
Redeemable Non-controlling Interest, start at Mar. 31, 2019	418,277,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	5,812,000				5,812,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	4,331,000
Foreign currency translation	2,000			2,000
Capital contributions	40,622,000
Capital withdrawals	(22,918,000)
Restricted stock awards issued, shares						585,383
Purchase of treasury stock, at cost, shares						(685,783)
Purchase of treasury stock, at cost	(10,838,000)	(10,838,000)
Preferred stock dividends	(1,698,000)							$ (1,698,000)
Amortization of share based compensation	9,364,000		9,364,000
Common stock, shares outstanding, end at Jun. 30, 2019						29,480,287.000
Balance, end at Jun. 30, 2019	$ 817,424,000	$ (254,357,000)	$ 1,093,898,000	$ (3,000)	$ (22,449,000)	$ 334,000	$ 1,000
Preferred stock, shares outstanding, end at Jun. 30, 2019	120,750						120,750
Redeemable Non-controlling Interest, end at Jun. 30, 2019	$ 440,312,000