Investments - Summary of Investments (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	18 Months Ended
	Mar. 31, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Investments owned				$ 1,015,181	$ 669,800
Long-term incentive compensation-related				67,761	78,154
Available-for-sale Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				45	364
Other Investment [Member]
Summary of Investment Holdings [Line Items]
Investments owned				7,522	7,878
Private Equity Funds [Member]
Summary of Investment Holdings [Line Items]
Private equity investment, ownership percentage	7.60%	7.60%
Proceeds from sale of private equity investment	$ 74,800
Receivable from sale of private equity investment	$ 10,700	$ 10,700
Receivable from sale of private equity investment, term in escrow		18 months
Gain on sale of investment		$ 75,300
Long-term Incentive Compensation-Related [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				50,935	59,150
Long-term Incentive Compensation-Related [Member] | Investments In Limited Partnership Hedge Funds [Member]
Summary of Investment Holdings [Line Items]
Investments owned				16,826	19,004
US Treasury Bill Securities [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				28,937	24,942
Seed Capital [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				211,279	406,322
Seed Capital [Member] | Investments In Limited Partnership Hedge Funds [Member]
Summary of Investment Holdings [Line Items]
Investments owned				23,704	20,082
Equities [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				6,602	43,584
Exchange Traded Options [Member] | Trading Securities [Member]
Summary of Investment Holdings [Line Items]
Investments owned				3,106	5,910
Consolidated Private Equity Fund [Member]
Summary of Investment Holdings [Line Items]
Investments owned				0	23,897
Private Equity [Member]
Summary of Investment Holdings [Line Items]
Investments owned				45,278	48,761
Seed Capital [Member]
Summary of Investment Holdings [Line Items]
Investments owned				500,000	478,000
Investments Held By Consolidated VIEs [Member]
Summary of Investment Holdings [Line Items]
Investments owned				550,850	0
Time Deposits [Member]
Summary of Investment Holdings [Line Items]
Investments owned				$ 70,097	$ 9,906
Scenario, Forecast [Member] | Private Equity Funds [Member]
Summary of Investment Holdings [Line Items]
Proceeds from sale of private equity investment			$ 85,500