Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) cowenfund	Dec. 31, 2019 USD ($)
Assets	$ 4,732,051	$ 5,162,025
Liabilities	3,491,006	3,866,575
Cumulative Effect of New Accounting Principle in Period of Adoption	10
Variable Interest Entity, Primary Beneficiary [Member]
Assets	613,700	685,400
Liabilities	$ 34,900	$ 24,900
Other investment companies
Number of Funds | cowenfund	6
Operating Company [Member] | Hedge Funds
Asset Management Fees, Percent Fee	2.00%
Operating Company [Member] | During investment period | Minimum
Asset Management Fees, Percent Fee	1.00%
Operating Company [Member] | During investment period | Maximum
Asset Management Fees, Percent Fee	2.00%
Operating Company [Member] | After investment period | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	0.50%
Operating Company [Member] | After investment period | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	2.00%
Operating Company [Member] | After investment period | Other Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	0.20%
Operating Company [Member] | After investment period | Other Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	1.00%
Asset Company | Real Estate Funds | Minimum
Asset Management Fees, Percent Fee	0.25%
Asset Company | Real Estate Funds | Maximum
Asset Management Fees, Percent Fee	1.50%