Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Investments owned	$ 505,637	$ 443,589
Long-term incentive compensation obligations	57,031	66,034
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	501,900	523,200
Amount of investment deconsolidated	100,000
U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	47,565	52,609
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	42,096	51,758
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	119,333	160,672
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	92,996	81,154
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	8,233	4,981
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	14,935	14,276
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	123,281	22,923
Increase in fair value	100,400
Private equity (seed capital)
Summary of Investment Holdings [Line Items]
Investments owned	36,967	38,186
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	8,793	5,138
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 11,438	$ 11,892