Commitments and Contingencies Schedules of Unfunded Commitments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) cowenfund
Unfunded Commitments
Other Commitments [Line Items]
Number of Real Estate Investments | cowenfund	6
Real Estate Funds
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 18.4	[1]
Real Estate Funds | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	4 years
Healthcare Royalty Partners
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 6.2	[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. [Member]
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 0.4
Formation 8 Partners Hardware Fund I, L.P. [Member] | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	7 years
Lagunita Biosciences, LLC
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 0.5
Lagunita Biosciences, LLC | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	3 years
Eclipse Fund II, L.P. [Member]
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 0.9
Eclipse Fund II, L.P. [Member] | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	8 years
Eclipse Continuity Fund I, L.P. [Member]
Other Commitments [Line Items]
Other commitments, unfunded amount	$ 0.6
Eclipse Continuity Fund I, L.P. [Member] | Commitment to Invest
Other Commitments [Line Items]
Term of capital commitment	9 years

[1] The Company had unfunded commitments pertaining to capital commitments in six real estate investments held by the Company, all of which pertain to related party investments. Such commitments can be called at any time up to four years, subject to advance notice.

[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.