Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Summary of Investment Holdings [Line Items]
Investments owned	$ 275,193	$ 714,344
Long-term incentive compensation obligations	49,717	52,429
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	381,400	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	36,137	38,883
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	84,697	105,951
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	53,193	73,409
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	3,046	2,568
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,580	13,546
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	63,723	67,153
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	8,634	8,783
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 12,183	$ 11,627