Summary of Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Investment commitments	$ 229,500
Limited Partnership Maximum Expiration Terms	12 years
Weighted-average remaining liquidation term (in years)	5 years 6 months
Limited Partnership Liquidation, Expected Initial Term (In Years)	10 years
Summary of Investment [Abstract]
Investment at fair value, cost	$ 738,951	$ 652,109
Cumulative unrealized gains (losses) on investments	401,643	240,155
Total investments	1,140,594	892,264
Entities with observable price change since January 1, 2018
Schedule of Investments [Line Items]
Investments in privately held entities that do not report NAV, impairment loss	17,100
Investments in publicly traded companies
Summary of Investment [Abstract]
Investment at fair value, cost	148,109	121,121
Cumulative unrealized gains (losses) on investments	170,528	62,884
Investment at fair value, book value	$ 318,637	184,005
Investments in privately held entities that report NAV
Schedule of Investments [Line Items]
Weighted-average remaining liquidation term (in years)	8 years 7 months 6 days
Summary of Investment [Abstract]
Investment at fair value, cost	$ 271,276	204,646
Cumulative unrealized gains (losses) on investments	162,626	113,159
Investment at fair value, book value	433,902	317,805
Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Investment in privately held entities that do not report fair value, book value	388,100	390,500
Investments in privately held entities that do not report NAV | Entities with observable price change since January 1, 2018
Schedule of Investments [Line Items]
Investment in privately held entities that do not report NAV, upward price adjustment	69,500
Investment in privately held entities that do not report NAV, downward price adjustment	(1,000)
Annual adjustments recognized on investments in privately held entities that do not report NAV	4,400
Investments in privately held entities that do not report NAV, annual upward price adjustment	5,200
Investments in privately held entities that do not report NAV, annual downward price adjustment	(832)
Summary of Investment [Abstract]
Investment at fair value, cost	42,045	39,421
Cumulative unrealized gains (losses) on investments	68,489	64,112
Investment in privately held entities that do not report fair value, book value	110,534	103,533
Investments in privately held entities that do not report NAV | Entities without observable price changes since January 1, 2018
Summary of Investment [Abstract]
Investment at fair value, cost	277,521	286,921
Cumulative unrealized gains (losses) on investments	0	0
Investment in privately held entities that do not report fair value, book value	$ 277,521	$ 286,921