Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 109,391	$ 144,819
Total deferred tax liabilities	(29,609)	(20,050)
Net deferred tax asset	79,782	124,769
Components of net deferred tax asset [Abstract]
Unearned premium reserves	13,862	16,209
Benefit plans	(19,142)	(9,444)
Loss reserves	1,921	1,785
Unrealized losses on investments	70,961	108,588
Deferred policy acquisition cost	(3,064)	(4,003)
Deferred compensation	7,466	6,806
Research and experimental costs	13,351	9,719
Other, net	(5,573)	(4,891)
Components of provisions for (benefit from) income taxes [Abstract]
Current federal	187,246	228,259	$ 161,055
Deferred federal	1,550	(5,235)	4,392
Other	484	1,661	1,347
Provision for income taxes	$ 189,280	$ 224,685	$ 166,794
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)	(0.50%)	(0.50%)	(0.60%)
Other, net (in hundredths)	0.50%	0.10%	0.40%
Effective income tax provision rate (in hundredths)	21.00%	20.60%	20.80%
Income taxes paid	$ 188,200	$ 236,500	$ 155,300
Amount of tax and loss bonds held	848,600
Unrecognized tax benefits	$ 0	$ 0