Acquisition (Pro Forma) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Revenues	$ 1,049,443	$ 966,916	$ 658,781
Net income (loss) attributable to Cowen Inc.	24,631	42,819	$ (60,882)
Quarton
Business Acquisition [Line Items]
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	1,200
Net revenues		922,001
Net income (loss)		$ 35,616
Basic (usd per share)		$ 1.20
Diluted (usd per share)		$ 1.15
Revenues	34,800
Net income (loss) attributable to Cowen Inc.	$ 5,700