Investments of Operating Entities and Consolidated Funds - Other Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Investment Holdings [Line Items]
Other investments	$ 141,548		$ 157,279
Portfolio Funds, Consolidated Funds	374,111		401,465
Portfolio Funds, at fair value
Investment Holdings [Line Items]
Other investments	100,148	[1]	120,023	[2]
Equity Method Investments
Investment Holdings [Line Items]
Other investments	41,099		36,991
Lehman claims, at fair value
Investment Holdings [Line Items]
Other investments	301		265
Enterprise LP [Member]
Investment Holdings [Line Items]
Portfolio Funds, Consolidated Funds	87,221		114,159
Merger Fund [Member]
Investment Holdings [Line Items]
Portfolio Funds, Consolidated Funds	286,890		281,572
Caerus LP [Member]
Investment Holdings [Line Items]
Portfolio Funds, Consolidated Funds	$ 0		$ 5,734

[1] In accordance with US GAAP, certain investments that are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated statement of financial condition.

[2] In accordance with US GAAP, certain investments that are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated statement of financial condition.