Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments owned	$ 519,753	$ 441,105
Long-term incentive compensation obligations	64,716	67,761
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	462,900	500,000
Private equity (seed capital)
Summary of Investment Holdings [Line Items]
Investments owned	38,553	45,278
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	52,090	70,097
Trading | U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	52,605	28,937
Trading | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	50,966	50,935
Trading | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	158,754	188,053
Trading | Equities
Summary of Investment Holdings [Line Items]
Investments owned	105,826	6,602
Trading | Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	9,368	3,106
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,750	16,826
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	25,740	23,704
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 12,101	$ 7,567