Investments - Summary of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments owned	$ 443,589	$ 441,105
Other Investment [Member]
Summary of Investment Holdings [Line Items]
Investments owned	11,991	7,567
US Treasury Bill Securities [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned	52,609	28,937
Long-term Incentive Compensation-Related [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned	51,758	50,935
Long-term Incentive Compensation-Related [Member] | Investments In Limited Partnership Hedge Funds [Member]
Summary of Investment Holdings [Line Items]
Investments owned	14,276	16,826
Seed Capital [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned	160,573	188,053
Seed Capital [Member] | Investments In Limited Partnership Hedge Funds [Member]
Summary of Investment Holdings [Line Items]
Investments owned	22,923	23,704
Equities [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned	81,154	6,602
Exchange Traded Options [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned	4,981	3,106
Private Equity [Member]
Summary of Investment Holdings [Line Items]
Investments owned	38,186	45,278
Seed Capital [Member]
Summary of Investment Holdings [Line Items]
Investments owned	523,200	500,000
Time Deposits [Member]
Summary of Investment Holdings [Line Items]
Investments owned	$ 5,138	$ 70,097