Schedule III-Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Supplementary insurance information
Deferred policy acquisition costs	$ 166,214	$ 146,566	$ 127,859
Unpaid losses and settlement expenses, gross	2,693,470	2,446,025	2,315,637
Unearned premiums, gross	984,140	892,326	785,085
Net premiums earned	1,526,406	1,294,306	1,144,436
Incurred losses and settlement expenses
Current accident year	834,562	712,960	636,955
Prior accident year	(95,309)	(108,547)	(122,579)
Policy acquisition costs	464,040	418,325	369,632
Other operating expenses	112,460	98,383	82,212
Net premiums written	1,605,521	1,427,747	1,241,536
Casualty Segment
Supplementary insurance information
Deferred policy acquisition costs	88,616	73,334	66,285
Unpaid losses and settlement expenses, gross	2,212,648	2,043,556	1,929,091
Unearned premiums, gross	564,772	491,479	466,178
Net premiums earned	852,837	758,346	711,832
Incurred losses and settlement expenses
Current accident year	577,368	496,530	468,661
Prior accident year	(52,878)	(78,498)	(87,225)
Policy acquisition costs	245,519	221,589	204,397
Other operating expenses	65,040	59,246	52,210
Net premiums written	915,625	788,982	744,607
Property segment
Supplementary insurance information
Deferred policy acquisition costs	48,302	46,366	36,767
Unpaid losses and settlement expenses, gross	337,787	301,907	293,737
Unearned premiums, gross	328,842	314,945	237,369
Net premiums earned	531,384	401,530	307,886
Incurred losses and settlement expenses
Current accident year	231,949	193,258	145,672
Prior accident year	(33,143)	(21,196)	(24,927)
Policy acquisition costs	131,171	116,344	95,203
Other operating expenses	33,871	26,808	19,416
Net premiums written	542,997	500,057	364,644
Surety Segment
Supplementary insurance information
Deferred policy acquisition costs	29,296	26,866	24,807
Unpaid losses and settlement expenses, gross	143,035	100,562	92,809
Unearned premiums, gross	90,526	85,902	81,538
Net premiums earned	142,185	134,430	124,718
Incurred losses and settlement expenses
Current accident year	25,245	23,172	22,622
Prior accident year	(9,288)	(8,853)	(10,427)
Policy acquisition costs	87,350	80,392	70,032
Other operating expenses	13,549	12,329	10,586
Net premiums written	$ 146,899	$ 138,708	$ 132,285