Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) lots	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) lots	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) lots
Narrative items:
Investable assets	$ 27,170,000		$ 27,170,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ (65,119)	$ 32,536	$ (69,328)	$ 79,403
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	3,730		3,730		2,230
Total number of positions (lots) | lots	10,540		10,540		9,590
Largest single loss	$ 1,200		$ 1,200		$ 900
Equity securities, at fair value	1,257,317		$ 1,257,317		$ 838,925
Minimum
Narrative items:
Time lag for reporting			1 month
Maximum
Narrative items:
Time lag for reporting			3 months
Underwriting segments
Narrative items:
Investable assets	24,530,000		$ 24,530,000
Equity in net (loss) income of investment funds accounted for using the equity method	(65,119)	32,536	(69,328)	79,403
Other
Narrative items:
Investable assets	2,640,000		2,640,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ 0	$ 0	$ 0	$ 0