Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Investments owned	$ 405,381	$ 443,589
Long-term incentive compensation obligations	56,400	66,034
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	563,300	523,200
U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	52,685	52,609
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	42,022	51,758
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	137,033	160,672
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	74,177	81,154
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	5,395	4,981
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	14,378	14,276
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	24,734	22,923
Private equity (seed capital)
Summary of Investment Holdings [Line Items]
Investments owned	38,011	38,186
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	5,160	5,138
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 11,786	$ 11,892