Investment Information - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) lots	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) lots
Narrative items:
Investable assets	$ 29,100,000
Equity in net (loss) income of investment funds accounted for using the equity method	71,686	$ (4,209)
Equity securities, at fair value	$ 1,532,906		$ 1,444,830
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	4,610		2,320
Total number of positions (lots) | lots	11,360		11,180
Largest single loss	$ 1,100		$ 900
Minimum
Narrative items:
Time lag for reporting	one
Maximum
Narrative items:
Time lag for reporting	three
Underwriting segments
Narrative items:
Investable assets	$ 26,300,000
Equity in net (loss) income of investment funds accounted for using the equity method	71,686	(4,209)
Other
Narrative items:
Investable assets	2,700,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ 0	$ 0