Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Computation of earnings per share:
Net income (loss)	$ 152,066	$ (72,111)
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds	4,562	(62,188)
Net income (loss) attributable to Cowen Inc.	147,504	(9,923)
Preferred stock dividends	1,698	1,698
Net income (loss) attributable to Cowen Inc. common stockholders	$ 145,806	$ (11,621)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	27,359,000	28,598,000
Weighted average shares used in diluted computation, shares	33,565,000	28,598,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 5.33	$ (0.41)
Earnings Per Share, Diluted (in dollars per share)	$ 4.34	$ (0.41)
2022 convertible note | Convertible Debt
Shares for basic and diluted calculations:
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities	2,659,000	0
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger	58.3324%
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	3,102,000	0
Restricted Stock Units (RSUs) | Equity Plans
Shares for basic and diluted calculations:
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number			334,230
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	445,000	0
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	26,852,331	27,628,117	26,845,628	28,610,357
Common stock, restricted shares, shares	259,536
Common Stock Class A | Restricted Stock
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	292,968		334,230