Investments Summary of Investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Investment commitments	$ 175,700
Limited partnership liquidation, expected initial term (in years)	10 years
Weighted average remaining liquidation term (in years)	4 years 7 months
Reclassification of cumulative net unrealized gains on non-real estate investments	$ (90,750)
Summary of Investment [Abstract]
Investment, at cost	511,162	$ 473,483
Cumulative unrealized gains (losses) on investments	213,148	49,771
Total investments	724,310	523,254
Investments in publicly traded companies [Abstract]
Investments in publicly traded companies, at cost	67,801	59,740
Investments in publicly traded companies	163,671	109,511
Investments in publicly traded companies
Summary of Investment [Abstract]
Cumulative unrealized gains (losses) on investments	95,870	49,771
Investment in privately held entities that report NAV
Schedule of Investments [Line Items]
Investment commitments	173,000
Summary of Investment [Abstract]
Investment, at cost	159,231	148,627
Cumulative unrealized gains (losses) on investments	106,235
Investments in privately held entities without readily determinable fair values	265,466	148,627
Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Investment, at cost		265,116
Investments in privately held entities without readily determinable fair values		$ 265,116
Accumulated Other Comprehensive Income
Schedule of Investments [Line Items]
Reclassification of cumulative net unrealized gains on non-real estate investments	49,771
Unrealized gains (losses) on available-for-sale equity securities
Schedule of Investments [Line Items]
Reclassification of cumulative net unrealized gains on non-real estate investments	49,771
Entities with observable price change since January 1, 2018 | Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Investment, at cost	23,491
Cumulative unrealized gains (losses) on investments	11,043
Investments in privately held entities without readily determinable fair values	34,534
Entities without observable price changes since January 1, 2018 | Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Investment, at cost	260,639
Cumulative unrealized gains (losses) on investments	0
Investments in privately held entities without readily determinable fair values	260,639
Upward adjustments for privately held entities that do not report NAV | Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Cumulative unrealized gains (losses) on investments	11,396
Downward adjustments for privately held entities that do no report NAV | Investments in privately held entities that do not report NAV
Summary of Investment [Abstract]
Cumulative unrealized gains (losses) on investments	$ (353)