Schedule III-Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Supplementary insurance information
Deferred policy acquisition costs	$ 127,859	$ 103,553	$ 88,425
Unpaid losses and settlement expenses, gross	2,315,637	2,043,555	1,750,049
Unearned premiums, gross	785,085	680,444	586,386
Net premiums earned	1,144,436	980,903	865,747
Incurred losses and settlement expenses
Current accident year	636,955	582,065	543,937
Prior accident years	(122,579)	(125,463)	(101,053)
Policy acquisition costs	369,632	317,468	286,438
Other operating expenses	82,212	76,907	66,828
Net premiums written	1,241,536	1,057,533	892,088
Casualty Segment
Supplementary insurance information
Deferred policy acquisition costs	66,285	55,760	48,255
Unpaid losses and settlement expenses, gross	1,929,091	1,760,469	1,567,544
Unearned premiums, gross	466,178	438,248	385,736
Net premiums earned	711,832	633,639	569,521
Incurred losses and settlement expenses
Current accident year	468,661	420,259	397,174
Prior accident years	(87,225)	(108,632)	(75,075)
Policy acquisition costs	204,397	179,354	162,058
Other operating expenses	52,210	47,139	40,937
Net premiums written	744,607	674,709	583,244
Property segment
Supplementary insurance information
Deferred policy acquisition costs	36,767	25,764	19,655
Unpaid losses and settlement expenses, gross	293,737	196,369	150,008
Unearned premiums, gross	237,369	168,209	131,274
Net premiums earned	307,886	231,837	183,720
Incurred losses and settlement expenses
Current accident year	145,672	140,905	124,375
Prior accident years	(24,927)	(10,981)	(13,019)
Policy acquisition costs	95,203	72,008	59,926
Other operating expenses	19,416	18,605	15,620
Net premiums written	364,644	262,816	196,603
Surety Segment
Supplementary insurance information
Deferred policy acquisition costs	24,807	22,029	20,515
Unpaid losses and settlement expenses, gross	92,809	86,717	32,497
Unearned premiums, gross	81,538	73,987	69,376
Net premiums earned	124,718	115,427	112,506
Incurred losses and settlement expenses
Current accident year	22,622	20,901	22,388
Prior accident years	(10,427)	(5,850)	(12,959)
Policy acquisition costs	70,032	66,106	64,454
Other operating expenses	10,586	11,163	10,271
Net premiums written	$ 132,285	$ 120,008	$ 112,241