Operating Segment Information - Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue
Net premiums earned	$ 407,695	$ 389,489	$ 1,207,944	$ 1,129,230
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Consolidated revenue	509,264	469,995	1,416,755	1,331,268
Less: Expenses
Losses and settlement expenses	187,998	202,118	549,814	513,741
Insurance operating expenses	30,215	28,868	86,683	84,892
Earnings before income taxes	157,245	116,825	392,391	380,119
Reconciliation of earnings before income taxes
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Interest expense on debt	(1,364)	(1,617)	(4,049)	(4,839)
General corporate expense	(5,045)	(3,994)	(12,747)	(13,144)
Equity in earnings of unconsolidated investees	1,540	1,238	7,055	7,653
Casualty Segment
Revenue
Net premiums earned	243,508	219,638	707,194	627,014
Property Segment
Revenue
Net premiums earned	126,689	133,266	389,897	396,774
Surety Segment
Revenue
Net premiums earned	37,498	36,585	110,853	105,442
Operating Segments
Revenue
Net premiums earned	407,695	389,489	1,207,944	1,129,230
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Consolidated revenue	509,264	469,995	1,416,755	1,331,268
Less: Expenses
Earnings before income taxes	60,545	40,692	193,321	188,411
Reconciliation of earnings before income taxes
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Operating Segments | Casualty Segment
Revenue
Net premiums earned	243,508	219,638	707,194	627,014
Consolidated revenue	243,508	219,638	707,194	627,014
Less: Expenses
Losses and settlement expenses	150,912	137,951	438,007	369,273
Amortization of deferred acquisition costs	47,335	41,240	135,999	117,979
Other policy acquisition costs	23,199	21,266	67,935	64,121
Insurance operating expenses	17,576	16,632	50,407	49,103
Earnings before income taxes	4,486	2,549	14,846	26,538
Reconciliation of earnings before income taxes
Depreciation and amortization expense	1,517	1,428	4,556	4,228
Operating Segments | Property Segment
Revenue
Net premiums earned	126,689	133,266	389,897	396,774
Consolidated revenue	126,689	133,266	389,897	396,774
Less: Expenses
Losses and settlement expenses	32,815	60,991	103,999	134,950
Amortization of deferred acquisition costs	26,311	27,054	79,985	80,319
Other policy acquisition costs	8,219	6,054	23,496	14,696
Insurance operating expenses	8,920	8,765	25,567	25,511
Earnings before income taxes	50,424	30,402	156,850	141,298
Reconciliation of earnings before income taxes
Depreciation and amortization expense	534	485	1,553	1,406
Operating Segments | Surety Segment
Revenue
Net premiums earned	37,498	36,585	110,853	105,442
Consolidated revenue	37,498	36,585	110,853	105,442
Less: Expenses
Losses and settlement expenses	4,271	3,176	7,808	9,518
Amortization of deferred acquisition costs	13,103	12,471	38,361	35,574
Other policy acquisition costs	10,770	9,726	32,350	29,497
Insurance operating expenses	3,719	3,471	10,709	10,278
Earnings before income taxes	5,635	7,741	21,625	20,575
Reconciliation of earnings before income taxes
Depreciation and amortization expense	370	243	1,039	755
Corporate
Revenue
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Reconciliation of earnings before income taxes
Net investment income	41,270	36,694	117,414	103,502
Net realized gains (losses)	18,318	5,420	48,234	11,222
Net unrealized gains on equity securities	41,981	38,392	43,163	87,314
Interest expense on debt	(1,364)	(1,617)	(4,049)	(4,839)
General corporate expense	(5,045)	(3,994)	(12,747)	(13,144)
Equity in earnings of unconsolidated investees	$ 1,540	$ 1,238	$ 7,055	$ 7,653