Investments of Operating Entities and Consolidated Funds - Securities loaned transactions by remaining contractual maturity and class of collateral pledged (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	$ 462,854	$ 456,831
Equities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	462,854	456,831
Equities | Open and Overnight
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	462,854	456,831
Equities | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	0	0
Equities | 31 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	0	0
Equities | Greater than 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Equities	$ 0	$ 0