Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Summary of Investment Holdings [Line Items]
Investments owned	$ 224,703	$ 714,344
Long-term incentive compensation obligations	48,906	52,429
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	398,900	391,600
U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	0	392,424
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	34,956	38,883
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	84,947	105,951
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	14,917	73,409
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	2,291	2,568
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,950	13,546
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	52,881	67,153
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	8,487	8,783
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 12,274	$ 11,627