Operating Segment Information - Summary by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenues
Net premiums earned	$ 379,065		$ 322,280		$ 739,741	$ 630,003
Net investment income	33,961		28,788		66,808	55,872
Net realized gains (losses)	(192)		5,580		5,802	20,200
Net unrealized gains on equity securities	3,608		25,214		48,922	40,710
Consolidated revenue	416,442		381,862		861,273	746,785
Net Earnings
Net underwriting income	70,024		41,201		147,719	109,091
Net investment income	33,961		28,788		66,808	55,872
Net realized gains (losses)	(192)		5,580		5,802	20,200
Net unrealized gains on equity securities	3,608		25,214		48,922	40,710
General corporate expense and interest on debt	(5,744)		(6,266)		(12,372)	(12,488)
Equity in earnings of unconsolidated investees	1,646		1,514		6,415	5,437
Earnings (loss) before income taxes	103,303		96,031		263,294	218,822
Income tax expense	21,311		18,379		53,402	42,359
Net earnings (loss)	81,992	$ 127,900	77,652	$ 98,811	209,892	176,463
Casualty Segment
Revenues
Net premiums earned	209,100		187,048		407,376	373,079
Net Earnings
Net underwriting income	10,315		6,977		23,989	38,808
Property Segment
Revenues
Net premiums earned	134,097		101,841		263,508	190,608
Net Earnings
Net underwriting income	53,180		25,877		110,896	54,260
Surety Segment
Revenues
Net premiums earned	35,868		33,391		68,857	66,316
Net Earnings
Net underwriting income	$ 6,529		$ 8,347		$ 12,834	$ 16,023