Commitments and Contingencies Schedules of Unfunded Commitments (Details) $ in Millions		12 Months Ended
		Dec. 31, 2018 USD ($) cowenfund	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Sublease income related to operating leases		$ 1.4	$ 1.1	$ 2.2
Unfunded Commitments
Other Commitments [Line Items]
Number of real estate investments | cowenfund		5
Real Estate Funds
Other Commitments [Line Items]
Other commitments, unfunded amount	[1]	$ 22.6
Healthcare Royalty Partners
Other Commitments [Line Items]
Other commitments, unfunded amount	[2]	$ 4.9
Healthcare Royalty Partners | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		2 years
Eclipse Ventures Fund I, L.P.
Other Commitments [Line Items]
Other commitments, unfunded amount		$ 0.2
Eclipse Ventures Fund I, L.P. | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		6 years
Lagunita Biosciences, LLC
Other Commitments [Line Items]
Other commitments, unfunded amount		$ 1.0
Lagunita Biosciences, LLC | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		3 years
Eclipse Fund II, L.P.
Other Commitments [Line Items]
Other commitments, unfunded amount		$ 0.8
Eclipse Fund II, L.P. | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		7 years
Eclipse Continuity Fund I, L.P.
Other Commitments [Line Items]
Other commitments, unfunded amount		$ 0.6
Eclipse Continuity Fund I, L.P. | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		8 years
Cowen Healthcare Investments II LP
Other Commitments [Line Items]
Other commitments, unfunded amount		$ 6.9
Cowen Healthcare Investments II LP | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		3 years
Minimum | Real Estate Funds | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		2 years
Maximum | Real Estate Funds | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment		4 years

[1] The Company had unfunded commitments pertaining to capital commitments in five real estate investments held by the Company, all of which pertain to related party investments. Such commitments can be called at any time between

[2] The Company is a limited partner of the HealthCare Royalty Partners funds (which are managed by Healthcare Royalty Management) and is a member of HealthCare Royalty Partners General Partners. The Company will make its pro-rata investment in the HealthCare Royalty Partners funds along with the other limited partners.