Operating Segment Information - Operating Results by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenue
Net premiums earned	$ 1,526,406	$ 1,294,306	$ 1,144,436
Net investment income	142,278	120,383	86,078
Net realized gains	19,966	32,518	588,515
Net unrealized gains (losses) on equity securities	81,734	64,787	(121,037)
Consolidated revenue	1,770,384	1,511,994	1,697,992
Less: Expenses
Losses and settlement expenses	739,253	604,413	514,376
Insurance operating expenses	112,460	98,383	82,212
Earnings before income taxes	427,551	377,265	720,678
Reconciliation of earnings before income taxes
Net investment income	142,278	120,383	86,078
Net realized gains	19,966	32,518	588,515
Net unrealized gains (losses) on equity securities	81,734	64,787	(121,037)
Interest expense on debt	(6,331)	(7,301)	(8,047)
General corporate expense	(15,880)	(15,917)	(12,900)
Equity in earnings of unconsolidated investees	(4,869)	9,610	9,853
Casualty Segment
Revenue
Net premiums earned	852,837	758,346	711,832
Property Segment
Revenue
Net premiums earned	531,384	401,530	307,886
Surety Segment
Revenue
Net premiums earned	142,185	134,430	124,718
Operating Segments
Less: Expenses
Earnings before income taxes	210,653	173,185	178,216
Operating Segments | Casualty Segment
Revenue
Net premiums earned	852,837	758,346	711,832
Consolidated revenue	852,837	758,346	711,832
Less: Expenses
Losses and settlement expenses	524,490	418,032	381,436
Amortization of deferred acquisition costs	161,532	138,968	126,048
Other policy acquisition costs	83,987	82,621	78,349
Insurance operating expenses	65,040	59,246	52,210
Earnings before income taxes	17,788	59,479	73,789
Reconciliation of earnings before income taxes
Depreciation and amortization expense	5,705	5,991	4,961
Operating Segments | Property Segment
Revenue
Net premiums earned	531,384	401,530	307,886
Consolidated revenue	531,384	401,530	307,886
Less: Expenses
Losses and settlement expenses	198,806	172,062	120,745
Amortization of deferred acquisition costs	108,235	94,173	67,450
Other policy acquisition costs	22,936	22,171	27,753
Insurance operating expenses	33,871	26,808	19,416
Earnings before income taxes	167,536	86,316	72,522
Reconciliation of earnings before income taxes
Depreciation and amortization expense	1,924	1,807	1,653
Operating Segments | Surety Segment
Revenue
Net premiums earned	142,185	134,430	124,718
Consolidated revenue	142,185	134,430	124,718
Less: Expenses
Losses and settlement expenses	15,957	14,319	12,195
Amortization of deferred acquisition costs	48,254	42,754	38,907
Other policy acquisition costs	39,096	37,638	31,125
Insurance operating expenses	13,549	12,329	10,586
Earnings before income taxes	25,329	27,390	31,905
Reconciliation of earnings before income taxes
Depreciation and amortization expense	991	1,648	1,705
Corporate
Revenue
Net investment income	142,278	120,383	86,078
Net realized gains	19,966	32,518	588,515
Net unrealized gains (losses) on equity securities	81,734	64,787	(121,037)
Reconciliation of earnings before income taxes
Net investment income	142,278	120,383	86,078
Net realized gains	19,966	32,518	588,515
Net unrealized gains (losses) on equity securities	81,734	64,787	(121,037)
Interest expense on debt	(6,331)	(7,301)	(8,047)
General corporate expense	(15,880)	(15,917)	(12,900)
Equity in earnings of unconsolidated investees	$ (4,869)	$ 9,610	$ 9,853