Investments - Summary of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Investments owned	$ 714,344	$ 443,589
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	391,600	523,200
U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	392,424	52,609
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	38,883	51,758
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	105,951	160,672
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	73,409	81,154
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	2,568	4,981
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,546	14,276
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	67,153	22,923
Private equity (seed capital)
Summary of Investment Holdings [Line Items]
Investments owned	0	38,186
Private equity (seed capital) | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned		37,200
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	8,783	5,138
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 11,627	$ 11,892