Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments owned	$ 479,981	$ 441,105
Long-term incentive compensation obligations	62,585	67,761
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	465,900	500,000
Private equity (seed capital)
Summary of Investment Holdings [Line Items]
Investments owned	44,852	45,278
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	50,429	70,097
Trading | U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	27,764	28,937
Trading | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	49,189	50,935
Trading | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	166,739	188,053
Trading | Equities
Summary of Investment Holdings [Line Items]
Investments owned	82,381	6,602
Trading | Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	8,348	3,106
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,396	16,826
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	25,283	23,704
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 11,600	$ 7,567