Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 144,819	$ 32,331
Total deferred tax liabilities	(20,050)	(71,743)
Net deferred tax asset	124,769
Net deferred tax liability		(39,412)
Components of net deferred tax asset [Abstract]
Unearned premium reserves	16,209	19,116
Benefit plans	(9,444)	(21,360)
Loss reserves	1,785	4,034
Unrealized depreciation in investments	108,588
Unrealized appreciation in investments		(39,883)
Deferred policy acquisition cost	(4,003)	(4,551)
Deferred compensation	6,806	6,118
Research and experimental costs	9,719	0
Other, net	(4,891)	(2,886)
Components of provisions for (benefit from) income taxes [Abstract]
Current federal	228,259	161,055	$ 85,574
Deferred federal	(5,235)	4,392	28,244
Other	1,661	1,347	(648)
Provision for income taxes	$ 224,685	$ 166,794	$ 113,170
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%)	(0.60%)	(0.90%)
Other, net (in hundredths)	0.10%	0.40%	0.10%
Effective income tax provision rate (in hundredths)	20.60%	20.80%	20.20%
Income taxes paid	$ 236,500	$ 155,300	$ 79,600
Amount of tax and loss bonds held	661,700
Unrecognized tax benefits	$ 0	$ 0