Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues
Net premiums earned	$ 318,409			$ 291,468			$ 948,412	$ 843,430
Net investment income	31,963			21,270			87,835	57,625
Net realized gains	6,558			573,170			26,758	591,562
Net unrealized gains (losses) on equity securities	(25,236)			(26,414)			15,474	(155,218)
Consolidated revenue	331,694			859,494			1,078,479	1,337,399
Net Earnings
Net underwriting income	4,247			8,796			113,338	124,230
Net investment income	31,963			21,270			87,835	57,625
Net realized gains	6,558			573,170			26,758	591,562
Net unrealized gains (losses) on equity securities	(25,236)			(26,414)			15,474	(155,218)
General corporate expense and interest on debt	(4,245)			(4,768)			(16,733)	(14,587)
Equity in earnings (loss) of unconsolidated investees	1,732			(17,352)			7,169	3,061
Earnings before income taxes	15,019			554,702			233,841	606,673
Income tax expense	1,483			114,809			43,842	121,096
Net earnings	13,536	$ 77,652	$ 98,811	439,893	$ (2,239)	$ 47,923	189,999	485,577
Casualty Segment
Revenues
Net premiums earned	189,305			179,615			562,384	528,494
Net Earnings
Net underwriting income	18,670			11,329			57,478	60,418
Property Segment
Revenues
Net premiums earned	94,988			80,844			285,596	222,974
Net Earnings
Net underwriting income	(20,671)			(8,308)			33,589	40,273
Surety Segment
Revenues
Net premiums earned	34,116			31,009			100,432	91,962
Net Earnings
Net underwriting income	$ 6,248			$ 5,775			$ 22,271	$ 23,539