Investments - Summary of Investments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule of Investments
Investments in privately held entities that do not report NAV, cumulative downward price adjustment	$ (900)		$ (900)
Investments in privately held entities that do not report NAV, cumulative impairment loss	(40,400)		(40,400)
Unrealized (losses) gains	(14,432)	$ (14,013)	183,348	$ 140,495
Investment commitments	345,800		$ 345,800
Limited partnership maximum expiration terms			12 years
Weighted-average remaining liquidation term (in years)			5 years
Limited partnership liquidation, expected initial term (in years)			10 years
Summary of Investment [Abstract]
Investment at fair value, cost	1,041,878		$ 1,041,878		$ 835,438
Cumulative unrealized gains (losses) on investments	929,833		929,833		775,676
Total investments held at adjusted carrying value or fair value	1,971,711		1,971,711
Investments accounted for under the equity method	75,167		75,167
Total investments	2,046,878		2,046,878		1,611,114
Investments in publicly traded companies
Summary of Investment [Abstract]
Investment at fair value, cost	282,816		282,816		208,754
Cumulative unrealized gains (losses) on investments	374,515		374,515		351,076
Investments at fair value, book value	657,331		$ 657,331		559,830
Investments in privately held entities that report NAV
Schedule of Investments
Weighted-average remaining liquidation term (in years)			8 years 10 months 24 days
Summary of Investment [Abstract]
Investment at fair value, cost	355,400		$ 355,400		334,341
Cumulative unrealized gains (losses) on investments	480,412		480,412		327,741
Investments at fair value, book value	835,812		835,812		662,082
Investments in privately held entities that do not report NAV | Entities with observable price change
Schedule of Investments
Investment in privately held entities that do not report NAV, cumulative price adjustments	34,500		34,500
Investments in privately held entities that do not report NAV, cumulative upward price adjustment	75,800		75,800
Annual adjustments recognized on investments in privately held entities that do not report NAV			(28,000)	(17,300)
Investments in privately held entities that do not report NAV, annual upward price adjustment			29,900	8,500
Investments in privately held entities that do not report NAV, annual downward price adjustment			57,900	25,800
Summary of Investment [Abstract]
Investment at fair value, cost	54,562		54,562		47,545
Cumulative unrealized gains (losses) on investments	74,906		74,906		96,859
Investments in privately held entities that do not report fair value, book value	129,468		129,468		144,404
Investments in privately held entities that do not report NAV | Entities without observable price changes
Summary of Investment [Abstract]
Investment at fair value, cost	349,100		349,100		244,798
Cumulative unrealized gains (losses) on investments	0		0		0
Investments in privately held entities that do not report fair value, book value	$ 349,100		349,100		$ 244,798
Total investments held
Schedule of Investments
Unrealized (losses) gains			$ 231,500	$ 151,700