Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 63,533				$ 83,082				$ 63,533	$ 83,082
Total deferred tax liabilities	(57,791)				(13,898)				(57,791)	(13,898)
Net deferred tax asset	5,742				69,184				5,742	69,184
Components of net deferred tax asset [Abstract]
Unearned premium reserves	30,487				31,808				30,487	31,808
Benefit plans	(10,790)				(5,047)				(10,790)	(5,047)
Loss reserves	2,175				3,113				2,175	3,113
Unrealized (appreciation) depreciation in investments	(36,822)				9,407				(36,822)	9,407
Mortgage investments	8,359				8,307				8,359	8,307
Deferred compensation	9,270				8,662				9,270	8,662
AMT credit carryforward	8,303				17,521				8,303	17,521
Other, net	(5,240)				(4,587)				(5,240)	(4,587)
Components of provisions for (benefit from) income taxes [Abstract]
Current Federal									162,911	(16,272)	$ 73,348
Deferred Federal									11,860	185,598	351,677
Other									(557)	4,727	3,710
Provision for income taxes	45,599	$ 46,186	$ 43,433	$ 38,996	36,963	$ 49,994	$ 50,708	$ 36,388	174,214	174,053	428,735
Income tax expense due to remeasurement of net deferred tax assets											133,000
Income taxes paid									158,300	$ 12,200	$ 22,000
Amount of tax and loss bonds held	176,000								$ 176,000
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)									21.00%	21.00%	35.00%
Additional income tax provision (benefit) related to the rate decrease included in the Tax Act (in hundredths)									0	0	0.170
Additional income tax provision related to IRS litigation (in hundredths)									0.00%	(0.30%)	3.70%
Tax exempt municipal bond interest (in hundredths)									(0.60%)	(0.70%)	(1.40%)
Other, net (in hundredths)									0.10%	0.60%	0.40%
Effective income tax provision rate (in hundredths)									20.50%	20.60%	54.70%
Unrecognized tax benefits [Roll Forward]
Balance at beginning of year				$ 0				$ 142,821	$ 0	$ 142,821	$ 108,245
Additions for tax positions of prior years										0	35,003
Reductions for tax positions of prior years										(3,070)	(427)
Settlements										(139,751)	0
Balance at end of year	$ 0				$ 0				$ 0	0	$ 142,821
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