Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Summary of Investment Holdings [Line Items]
Investments owned	$ 262,508	$ 266,794
Long-term incentive compensation obligations	37,300	50,900
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	340,800	358,100
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	25,655	36,665
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	70,361	70,464
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	58,423	73,202
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	36,783	6,931
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	11,661	14,237
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	30,732	33,124
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	17,303	18,281
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 11,590	$ 13,890