Summary of Investments (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments
Investment commitments	$ 233,800
Limited partnership maximum expiration terms	11 years
Weighted-average remaining liquidation term (in years)	8 years 7 months
Limited partnership liquidation, expected initial term (in years)	10 years
Summary of Investment [Abstract]
Investment at fair value, cost	$ 688,543	$ 652,109
Cumulative unrealized gains (losses) on investments	312,361	240,155
Total investments	1,000,904	892,264
Investments in publicly traded companies
Summary of Investment [Abstract]
Investment at fair value, cost	127,760	121,121
Cumulative unrealized gains (losses) on investments	97,194	62,884
Investments at fair value, book value	224,954	184,005
Investments in privately held entities that report NAV
Schedule of Investments
Investment commitments	$ 233,300
Weighted-average remaining liquidation term (in years)	8 years 8 months
Summary of Investment [Abstract]
Investment at fair value, cost	$ 223,986	204,646
Cumulative unrealized gains (losses) on investments	145,616	113,159
Investments at fair value, book value	369,602	317,805
Investments in privately held entities that do not report NAV \| Entities with observable price change
Schedule of Investments
Investments in privately held entities that do not report NAV, cumulative upward price adjustment	69,800
Investments in privately held entities that do not report NAV, cumulative downward price adjustment	(247)
Annual adjustments recognized on investments in privately held entities that do not report NAV	5,400
Investments in privately held entities that do not report NAV, annual upward price adjustment	5,500
Investments in privately held entities that do not report NAV, annual downward price adjustment	(47)
Summary of Investment [Abstract]
Investment at fair value, cost	42,865	39,421
Cumulative unrealized gains (losses) on investments	69,551	64,112
Investments in privately held entities that do not report fair value, book value	112,416	103,533
Investments in privately held entities that do not report NAV \| Entities without observable price changes
Summary of Investment [Abstract]
Investment at fair value, cost	293,932	286,921
Cumulative unrealized gains (losses) on investments	0	0
Investments in privately held entities that do not report fair value, book value	$ 293,932	$ 286,921

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