Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	9 Months Ended
Sep. 30, 2020 USD ($) cowenfund	Dec. 31, 2019 USD ($)
Assets	$ 5,790,718	$ 5,162,025
Liabilities	4,609,517	3,866,575
Cumulative Effect of New Accounting Principle in Period of Adoption	10
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(10)
Variable Interest Entity, Primary Beneficiary [Member]
Assets	449,000	685,400
Liabilities	$ 6,600	$ 24,900
Other investment companies
Number of Funds \| cowenfund	5
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%

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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