Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) cowenfund	Dec. 31, 2020 USD ($) cowenfund
Assets	$ 9,946,796	$ 7,828,199
Liabilities	8,706,816	6,659,078
Variable Interest Entity, Primary Beneficiary [Member]
Assets	243,300	325,500
Liabilities	$ 8,000	$ 10,100
Other investment companies
Number of Funds | cowenfund	2	3
Operating Company | Hedge Funds
Asset Management Fees, Percent Fee	2.00%
Operating Company | During investment period | Minimum
Asset Management Fees, Percent Fee	1.00%
Operating Company | During investment period | Maximum
Asset Management Fees, Percent Fee	2.00%
Operating Company | After investment period | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	1.00%
Operating Company | After investment period | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	2.00%
Operating Company | After investment period | Other Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	0.20%
Operating Company | After investment period | Other Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	1.00%