Investments - Summary of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Summary of Investment Holdings [Line Items]
Investments owned	$ 253,692	$ 266,794
Long-term incentive compensation obligations	55,100	50,900
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	276,200	358,100
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	30,911	36,665
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	75,079	70,464
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	60,147	73,202
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	12,153	6,931
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	24,150	14,237
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	17,309	33,124
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	19,900	18,281
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 14,043	$ 13,890