Operating Segment Information - Summary by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues
Net premiums earned	$ 389,489			$ 318,409			$ 1,129,230	$ 948,412
Net investment income	36,694			31,963			103,502	87,835
Net realized gains	5,420			6,558			11,222	26,758
Net unrealized gains (losses) on equity securities	38,392			(25,236)			87,314	15,474
Consolidated revenue	469,995			331,694			1,331,268	1,078,479
Net Earnings
Net underwriting income	40,692			4,247			188,411	113,338
Net investment income	36,694			31,963			103,502	87,835
Net realized gains	5,420			6,558			11,222	26,758
Net unrealized gains (losses) on equity securities	38,392			(25,236)			87,314	15,474
General corporate expense and interest on debt	(5,611)			(4,245)			(17,983)	(16,733)
Equity in earnings of unconsolidated investees	1,238			1,732			7,653	7,169
Earnings (loss) before income taxes	116,825			15,019			380,119	233,841
Income tax expense	21,798			1,483			75,200	43,842
Net earnings (loss)	95,027	$ 81,992	$ 127,900	13,536	$ 77,652	$ 98,811	304,919	189,999
Casualty Segment
Revenues
Net premiums earned	219,638			189,305			627,014	562,384
Net Earnings
Net underwriting income	2,549			18,670			26,538	57,478
Property Segment
Revenues
Net premiums earned	133,266			94,988			396,774	285,596
Net Earnings
Net underwriting income	30,402			(20,671)			141,298	33,589
Surety Segment
Revenues
Net premiums earned	36,585			34,116			105,442	100,432
Net Earnings
Net underwriting income	$ 7,741			$ 6,248			$ 20,575	$ 22,271