Investment Information - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) lots	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) lots
Narrative items:
Investable assets	$ 24,880,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ (4,209)	$ 46,867
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	5,420		2,230
Total number of positions (lots) | lots	9,960		9,590
Largest single loss	$ 6,300		$ 900
Equity securities, at fair value	$ 1,181,903		$ 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	$ 22,380,000
Equity in net (loss) income of investment funds accounted for using the equity method	(4,209)	46,867
Other
Narrative items:
Investable assets	2,500,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ 0	$ 0