Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Summary of Investment Holdings [Line Items]
Investments owned	$ 323,205	$ 714,344
Long-term incentive compensation obligations	48,776	52,429
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	379,800	391,600
U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	74,342	392,424
Equity securities | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	35,130	38,883
Equity securities | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	73,677	105,951
Equity securities | Other
Summary of Investment Holdings [Line Items]
Investments owned	42,518	73,409
Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	5,803	2,568
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	13,646	13,546
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	46,927	67,153
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	18,070	8,783
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 13,092	$ 11,627