Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Computation of earnings per share:
Net income (loss)	$ 20,778	$ 9,828	$ 78,793	$ 50,502
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	5,290	5,162	41,053	35,412
Net income (loss) attributable to Cowen Inc.	15,488	4,666	37,740	15,090
Preferred stock dividends	1,698	1,698	5,094	5,094
Net income (loss) attributable to Cowen Inc. common stockholders	$ 13,790	$ 2,968	$ 32,646	$ 9,996
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,610,000	31,271,000	29,662,000	29,004,000
Weighted average shares used in diluted computation, shares	30,844,000	32,246,000	30,613,000	30,011,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.47	$ 0.09	$ 1.10	$ 0.34
Earnings Per Share, Diluted (in dollars per share)	$ 0.45	$ 0.09	$ 1.07	$ 0.33
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares		0	0	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	20,000	0	15,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	1,234,000	955,000	951,000	992,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	29,142,547	31,087,458	29,142,547	31,087,458	29,632,020	26,731,289.000
Common stock, restricted shares, shares	253,772		253,772		191,962