Commitments and Contingencies Schedules of Unfunded Commitments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) cowenfund
Unfunded Commitments
Other Commitments [Line Items]
Number of real estate investments | cowenfund	4
Real Estate Funds
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 4.6	[1]
Healthcare Royalty Partners
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 1.1	[2]
Healthcare Royalty Partners | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	2 years
Formation 8 Partners Hardware Fund I, L.P.
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 0.4
Formation 8 Partners Hardware 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	$ 9.6
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 four real estate investments held by the Company, all of which pertain to related party investments.

[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.