Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Computation of earnings per share:
Net income (loss)	$ 29,516	$ 6,583	$ 104,353	$ 26,596
Net Income (Loss) Attributable to Noncontrolling Interest	9,232	2,770	(19,843)	7,188
Net income (loss) attributable to Cowen Inc.	20,284	3,813	124,196	19,408
Preferred stock dividends	1,698	1,698	5,094	5,094
Net income (loss) attributable to Cowen Inc. common stockholders	$ 18,586	$ 2,115	$ 119,102	$ 14,314
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	27,663,000	29,529,000	28,078,000	29,687,000
Weighted average shares used in diluted computation, shares	29,970,000	31,264,000	29,646,000	31,381,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.67	$ 0.07	$ 4.24	$ 0.48
Earnings Per Share, Diluted (in dollars per share)	$ 0.62	$ 0.07	$ 4.02	$ 0.46
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	2,285,000	1,735,000	1,546,000	1,694,000
Restricted Stock | Quarton
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	22,000	0	22,000	0
Restricted Stock Units (RSUs) | Non-employee Director | Equity Plans
Earnings (loss) per share:
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	259,536		259,536			216,912
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	26,569,335	29,013,137	26,569,335	29,013,137	27,641,600	28,610,357	29,480,287	28,437,860
Common stock, restricted shares, shares	334,230		334,230			216,912