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Summary of Investments (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Schedule of Investments    
Investment commitments $ 231,600  
Limited partnership maximum expiration terms 12 years  
Weighted-average remaining liquidation term (in years) 5 years 7 months 6 days  
Limited partnership liquidation, expected initial term (in years) 10 years  
Summary of Investment [Abstract]    
Investment at fair value, cost $ 737,078 $ 652,109
Cumulative unrealized gains (losses) on investments 253,376 240,155
Total investments 990,454 892,264
Investments in publicly traded companies    
Summary of Investment [Abstract]    
Investment at fair value, cost 173,063 121,121
Cumulative unrealized gains (losses) on investments 39,956 62,884
Investments at fair value, book value 213,019 184,005
Investments in privately held entities that report NAV    
Schedule of Investments    
Investment commitments $ 231,000  
Weighted-average remaining liquidation term (in years) 8 years 7 months 6 days  
Summary of Investment [Abstract]    
Investment at fair value, cost $ 253,696 204,646
Cumulative unrealized gains (losses) on investments 139,608 113,159
Investments at fair value, book value 393,304 317,805
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 74,500  
Investments in privately held entities that do not report NAV, cumulative downward price adjustment (730)  
Annual adjustments recognized on investments in privately held entities that do not report NAV 9,700  
Investments in privately held entities that do not report NAV, annual upward price adjustment 10,200  
Investments in privately held entities that do not report NAV, annual downward price adjustment (530)  
Summary of Investment [Abstract]    
Investment at fair value, cost 42,017 39,421
Cumulative unrealized gains (losses) on investments 73,812 64,112
Investments in privately held entities that do not report fair value, book value 115,829 103,533
Investments in privately held entities that do not report NAV | Entities without observable price changes    
Summary of Investment [Abstract]    
Investment at fair value, cost 268,302 286,921
Cumulative unrealized gains (losses) on investments 0 0
Investments in privately held entities that do not report fair value, book value $ 268,302 $ 286,921