Schedule III-Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplementary insurance information
Deferred policy acquisition costs	$ 146,566	$ 127,859	$ 103,553
Unpaid losses and settlement expenses, gross	2,446,025	2,315,637	2,043,555
Unearned premiums, gross	892,326	785,085	680,444
Net premiums earned	1,294,306	1,144,436	980,903
Incurred losses and settlement expenses
Current accident year	712,960	636,955	582,065
Prior accident years	(108,547)	(122,579)	(125,463)
Policy acquisition costs	418,325	369,632	317,468
Other operating expenses	98,383	82,212	76,907
Net premiums written	1,427,747	1,241,536	1,057,533
Casualty Segment
Supplementary insurance information
Deferred policy acquisition costs	73,334	66,285	55,760
Unpaid losses and settlement expenses, gross	2,043,556	1,929,091	1,760,469
Unearned premiums, gross	491,479	466,178	438,248
Net premiums earned	758,346	711,832	633,639
Incurred losses and settlement expenses
Current accident year	496,530	468,661	420,259
Prior accident years	(78,498)	(87,225)	(108,632)
Policy acquisition costs	221,589	204,397	179,354
Other operating expenses	59,246	52,210	47,139
Net premiums written	788,982	744,607	674,709
Property segment
Supplementary insurance information
Deferred policy acquisition costs	46,366	36,767	25,764
Unpaid losses and settlement expenses, gross	301,907	293,737	196,369
Unearned premiums, gross	314,945	237,369	168,209
Net premiums earned	401,530	307,886	231,837
Incurred losses and settlement expenses
Current accident year	193,258	145,672	140,905
Prior accident years	(21,196)	(24,927)	(10,981)
Policy acquisition costs	116,344	95,203	72,008
Other operating expenses	26,808	19,416	18,605
Net premiums written	500,057	364,644	262,816
Surety Segment
Supplementary insurance information
Deferred policy acquisition costs	26,866	24,807	22,029
Unpaid losses and settlement expenses, gross	100,562	92,809	86,717
Unearned premiums, gross	85,902	81,538	73,987
Net premiums earned	134,430	124,718	115,427
Incurred losses and settlement expenses
Current accident year	23,172	22,622	20,901
Prior accident years	(8,853)	(10,427)	(5,850)
Policy acquisition costs	80,392	70,032	66,106
Other operating expenses	12,329	10,586	11,163
Net premiums written	$ 138,708	$ 132,285	$ 120,008