Operating Segment Information - Summary of Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues
Net premiums earned	$ 1,144,436	$ 980,903	$ 865,747
Net investment income	86,078	68,862	67,893
Net realized gains	588,515	64,222	17,885
Net unrealized gains (losses) on equity securities	(121,037)	65,258	32,101
Consolidated revenue	1,697,992	1,179,245	983,626
INSURANCE EXPENSES
Loss and settlement expenses	514,376	456,602	442,884
Policy acquisition costs	369,632	317,468	286,438
Other insurance expenses	82,212	76,907	66,828
Insurance expenses	966,220	850,977	796,150
Net Earnings
Net underwriting income	178,216	129,926	69,597
Net investment income	86,078	68,862	67,893
Net realized gains	588,515	64,222	17,885
Net unrealized gains (losses) on equity securities	(121,037)	65,258	32,101
Interest on debt	(8,047)	(7,677)	(7,603)
General corporate expense	(12,900)	(13,330)	(10,265)
Equity in earnings of unconsolidated investees	9,853	37,060	20,233
Earnings (losses) before income taxes	720,678	344,321	189,841
Income tax expense	137,267	64,967	32,750
Net earnings (losses)	583,411	279,354	157,091
Casualty Segment
Revenues
Net premiums earned	711,832	633,639	569,521
INSURANCE EXPENSES
Loss and settlement expenses	381,436	311,627	322,099
Policy acquisition costs	204,397	179,354	162,058
Other insurance expenses	52,210	47,139	40,937
Net Earnings
Net underwriting income	73,789	95,519	44,427
Property segment
Revenues
Net premiums earned	307,886	231,837	183,720
INSURANCE EXPENSES
Loss and settlement expenses	120,745	129,924	111,356
Policy acquisition costs	95,203	72,008	59,926
Other insurance expenses	19,416	18,605	15,620
Net Earnings
Net underwriting income	72,522	11,300	(3,182)
Surety Segment
Revenues
Net premiums earned	124,718	115,427	112,506
INSURANCE EXPENSES
Loss and settlement expenses	12,195	15,051	9,429
Policy acquisition costs	70,032	66,106	64,454
Other insurance expenses	10,586	11,163	10,271
Net Earnings
Net underwriting income	$ 31,905	$ 23,107	$ 28,352