Investments of Operating Entities and Consolidated Funds - Securities loaned transactions by remaining contractual maturity and class of collateral pledged (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	$ 456,831	$ 0
Stocks
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	456,831
Stocks | Open and Overnight
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	456,831
Stocks | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	0
Stocks | 31 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	0
Stocks | Greater than 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Stocks	$ 0