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)	Common Stock Class A	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Balance at Dec. 31, 2016	$ 772,650	$ (153,845)	$ 928,646	$ (2)	$ (2,442)	$ 292	$ 1
Common Stock, Shares, Outstanding at Dec. 31, 2016						26,731,289.000
Preferred stock, shares outstanding at Dec. 31, 2016							120,750
Redeemable Non-controlling Interest at Dec. 31, 2016	379,205
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	2,984				2,984
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	9,105
Foreign currency translation	(2)			(2)
Capital contributions	16,099
Capital withdrawals	(10,277)
Restricted stock awards issued, shares						909,442
Purchase of treasury stock, at cost, shares						(328,238)
Purchase of treasury stock, at cost	(4,455)	(4,455)
Preferred stock dividends (See Note 18)	(1,698)							$ (1,698)
Amortization of share based compensation	8,779		8,779
Balance at Mar. 31, 2017	778,258	(158,300)	937,425	(4)	(1,156)	$ 292	$ 1
Common Stock, Shares, Outstanding at Mar. 31, 2017						27,312,493
Preferred stock, shares outstanding at Mar. 31, 2017							120,750
Redeemable Non-controlling Interest at Mar. 31, 2017	394,132
Cumulative effect of the adoption of the new revenue recognition standard (see Note 3)	(559)
Balance at Dec. 31, 2017	$ 748,019	(186,846)	1,004,664	(8)	(70,116)	$ 324	$ 1
Common Stock, Shares, Outstanding at Dec. 31, 2017						29,632,020
Preferred stock, shares outstanding at Dec. 31, 2017	120,750						120,750
Redeemable Non-controlling Interest at Dec. 31, 2017	$ 440,604
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	16,853				16,853
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	11,156
Foreign currency translation	1			1
Capital contributions	8,259
Capital withdrawals	(35,134)
Deconsolidation of funds	(32,559)
Restricted stock awards issued, shares						881,032
Purchase of treasury stock, at cost, shares						(995,834)
Purchase of treasury stock, at cost	(14,069)	(14,069)
Preferred stock dividends (See Note 18)	(1,698)							$ (1,698)
Amortization of share based compensation	10,240		10,240
Balance at Mar. 31, 2018	$ 758,787	$ (200,915)	$ 1,014,904	$ (7)	$ (55,520)	$ 324	$ 1
Common Stock, Shares, Outstanding at Mar. 31, 2018						29,517,218
Preferred stock, shares outstanding at Mar. 31, 2018	120,750						120,750
Redeemable Non-controlling Interest at Mar. 31, 2018	$ 392,326