Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Computation of earnings per share:
Net income (loss)	$ 10,143	$ 30,006	$ 22,132	$ 58,015
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	4,331	24,607	6,537	35,763
Net income (loss) attributable to Cowen Inc.	5,812	5,399	15,595	22,252
Preferred stock dividends	1,698	1,698	3,396	3,396
Net income (loss) attributable to Cowen Inc. common stockholders	$ 4,114	$ 3,701	$ 12,199	$ 18,856
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	29,769,000	29,769,000	29,766,000	29,688,000
Weighted average shares used in diluted computation, shares	31,522,000	30,720,000	31,572,000	30,460,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.14	$ 0.12	$ 0.41	$ 0.64
Earnings Per Share, Diluted (in dollars per share)	$ 0.13	$ 0.12	$ 0.39	$ 0.62
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	1,753,000	951,000	1,806,000	772,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	29,480,287.000	29,609,472	29,480,287.000	29,609,472	29,580,687	28,437,860	29,517,000	29,632,020.00
Common stock, restricted shares, shares	174,285		174,285			253,772