Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Computation of earnings per share:
Net income (loss)	$ 28,009	$ 12,089
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	11,156	9,105
Net income (loss) attributable to Cowen Inc.	16,853	2,984
Preferred stock dividends	1,698	1,698
Net income (loss) attributable to Cowen Inc. common stockholders	$ 15,155	$ 1,286
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,625,000	27,061,000
Weighted average shares used in diluted computation, shares	30,492,000	28,401,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.51	$ 0.05
Earnings Per Share, Diluted (in dollars per share)	$ 0.50	$ 0.05
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	12,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	867,000	1,328,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	29,517,218	27,312,493	29,632,020	26,731,289.000
Common stock, restricted shares, shares	191,962		191,962