Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues
Net premiums earned	$ 322,280		$ 282,810		$ 630,003	$ 551,962
Net investment income	28,788		18,472		55,872	36,355
Net realized gains	5,580		12,804		20,200	18,392
Net unrealized gains (losses) on equity securities	25,214		(100,994)		40,710	(128,804)
Consolidated revenue	381,862		213,092		746,785	477,905
Net Earnings
Net underwriting income	41,201		55,956		109,091	115,434
Net investment income	28,788		18,472		55,872	36,355
Net realized gains	5,580		12,804		20,200	18,392
Net unrealized gains (losses) on equity securities	25,214		(100,994)		40,710	(128,804)
General corporate expense and interest on debt	(6,266)		(4,446)		(12,488)	(9,819)
Equity in earnings of unconsolidated investees	1,514		11,654		5,437	20,413
Earnings (loss) before income taxes	96,031		(6,554)		218,822	51,971
Income tax expense (benefit)	18,379		(4,315)		42,359	6,287
Net earnings (loss)	77,652	$ 98,811	(2,239)	$ 47,923	176,463	45,684
Casualty Segment
Revenues
Net premiums earned	187,048		177,123		373,079	348,879
Net Earnings
Net underwriting income	6,977		21,442		38,808	49,089
Property Segment
Revenues
Net premiums earned	101,841		74,690		190,608	142,130
Net Earnings
Net underwriting income	25,877		26,105		54,260	48,581
Surety Segment
Revenues
Net premiums earned	33,391		30,997		66,316	60,953
Net Earnings
Net underwriting income	$ 8,347		$ 8,409		$ 16,023	$ 17,764