Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments owned	$ 437,052	$ 441,105
Long-term incentive compensation obligations	61,570	67,761
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	533,500	500,000
Private equity
Summary of Investment Holdings [Line Items]
Investments owned	44,405	45,278
Time deposits
Summary of Investment Holdings [Line Items]
Investments owned	70,466	70,097
Trading | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	46,511	50,935
Trading | U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	23,974	28,937
Trading | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	194,670	188,053
Trading | Equities
Summary of Investment Holdings [Line Items]
Investments owned	5,246	6,602
Trading | Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	1,558	3,106
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	15,059	16,826
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	27,719	23,704
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 7,444	$ 7,567