Investments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Long-term incentive compensation-related	$ 66,034,000	$ 67,761,000
Investments owned	443,589,000	441,105,000
Proceeds from sales of available-for-sale investments	11,000	372,000	$ 4,240,000
Realized gains from sales of available-for-sale investments	0	0	1,300,000
Realized losses from sale of available-for-sale investments	0	0	$ 0
Seed Capital [Member]
Investment Holdings [Line Items]
Investments owned	$ 523,200,000	$ 500,000,000