Operating Segment Information - Summary of Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
REVENUES
Net premiums earned	$ 980,903	$ 865,747	$ 839,111
Net investment income	68,862	67,893	68,870
Net realized gains	64,222	17,885	17,520
Net unrealized gains on equity securities	65,258	32,101	78,090
Consolidated revenue	1,179,245	983,626	1,003,591
INSURANCE EXPENSES
Loss and settlement expenses	456,602	442,884	413,416
Policy acquisition costs	317,468	286,438	288,697
Other insurance expenses	76,907	66,828	69,430
Insurance expenses	850,977	796,150	771,543
NET EARNINGS
Net underwriting income	129,926	69,597	67,568
Net investment income	68,862	67,893	68,870
Net realized gains	64,222	17,885	17,520
Net unrealized gains on equity securities	65,258	32,101	78,090
Interest on debt	(7,677)	(7,603)	(7,588)
General corporate expense	(13,330)	(10,265)	(12,686)
Equity in earnings of unconsolidated investees	37,060	20,233	20,960
Earnings before income taxes	344,321	189,841	232,734
Income tax expense	64,967	32,750	41,092
Net earnings	279,354	157,091	191,642
Casualty Segment
REVENUES
Net premiums earned	633,639	569,521	558,458
INSURANCE EXPENSES
Loss and settlement expenses	311,627	322,099	330,156
Policy acquisition costs	179,354	162,058	166,499
Other insurance expenses	47,139	40,937	41,202
NET EARNINGS
Net underwriting income	95,519	44,427	20,601
Property segment
REVENUES
Net premiums earned	231,837	183,720	164,022
INSURANCE EXPENSES
Loss and settlement expenses	129,924	111,356	73,614
Policy acquisition costs	72,008	59,926	55,986
Other insurance expenses	18,605	15,620	16,279
NET EARNINGS
Net underwriting income	11,300	(3,182)	18,143
Surety Segment
REVENUES
Net premiums earned	115,427	112,506	116,631
INSURANCE EXPENSES
Loss and settlement expenses	15,051	9,429	9,646
Policy acquisition costs	66,106	64,454	66,212
Other insurance expenses	11,163	10,271	11,949
NET EARNINGS
Net underwriting income	$ 23,107	$ 28,352	$ 28,824