Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 38,443				$ 63,533				$ 38,443	$ 63,533
Total deferred tax liabilities	(98,485)				(57,791)				(98,485)	(57,791)
Net deferred tax (liability) asset					5,742					5,742
Deferred Tax Liabilities, Net	(60,042)								(60,042)
Components of net deferred tax asset [Abstract]
Unearned premium reserves	23,163				30,487				23,163	30,487
Benefit plans	(13,977)				(10,790)				(13,977)	(10,790)
Loss reserves	3,542				2,175				3,542	2,175
Unrealized appreciation in investments	(72,341)				(36,822)				(72,341)	(36,822)
Mortgage investments	0				8,359				0	8,359
Deferred compensation	6,776				9,270				6,776	9,270
AMT credit carryforward	0				8,303				0	8,303
Other, net	(7,205)				(5,240)				(7,205)	(5,240)
Components of provisions for (benefit from) income taxes [Abstract]
Current Federal									85,574	162,911	$ (16,272)
Deferred Federal									28,244	11,860	185,598
Other									(648)	(557)	4,727
Provision for income taxes	38,782	$ 33,518	$ 2,436	$ 38,434	45,599	$ 46,186	$ 43,433	$ 38,996	113,170	174,214	174,053
Income taxes paid									79,600	$ 158,300	$ 12,200
Amount of tax and loss bonds held	271,000								$ 271,000
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)									21.00%	21.00%	21.00%
Additional income tax provision related to IRS litigation (in hundredths)									0.00%	0.00%	(0.30%)
Tax exempt municipal bond interest (in hundredths)									(0.90%)	(0.60%)	(0.70%)
Other, net (in hundredths)									0.10%	0.10%	0.60%
Effective income tax provision rate (in hundredths)									20.20%	20.50%	20.60%
Unrecognized tax benefits	$ 0				$ 0				$ 0	$ 0
IRS
Reconciliation of effective income tax rate [Abstract]
Tax and interest payments for IRS settlement											$ 14,800
State tax authorities
Reconciliation of effective income tax rate [Abstract]
Tax and interest payments for IRS settlement											$ 36,800