Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 105,437	$ 109,391
Total deferred tax liabilities	(35,562)	(29,609)
Net deferred tax asset (liability)	69,875	79,782
Components of net deferred tax asset [Abstract]
Unearned premium reserves	11,111	13,862
Benefit plans	(25,195)	(19,142)
Loss reserves	2,291	1,921
Unrealized losses on investments	68,550	70,961
Deferred policy acquisition cost	(2,456)	(3,064)
Deferred compensation	7,878	7,466
Research and experimental costs	14,143	13,351
Other, net	(6,447)	(5,573)
Components of provisions for (benefit from) income taxes [Abstract]
Current federal	199,090	187,246	$ 228,259
Deferred federal	1,805	1,550	(5,235)
Other	4,820	484	1,661
Provision for income taxes	$ 205,715	$ 189,280	$ 224,685
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)	21.00%	21.00%	21.00%
Tax exempt municipal bond interest (in hundredths)	(0.30%)	(0.50%)	(0.50%)
Other, net (in hundredths)	0.50%	0.50%	0.10%
Effective income tax provision rate (in hundredths)	21.20%	21.00%	20.60%
Income taxes paid	$ 181,800	$ 188,200	$ 236,500
Transferrable federal tax credits purchased	21,400
Amount of tax and loss bonds held	968,200
Unrecognized tax benefits	$ 0	$ 0