Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 83,082				$ 258,663				$ 83,082	$ 258,663
Total deferred tax liabilities	(13,898)				(24,282)				(13,898)	(24,282)
Net deferred tax asset	69,184				234,381				69,184	234,381
Components of net deferred tax asset [Abstract]
Unearned premium reserves	31,808				29,196				31,808	29,196
Benefit plans	(5,047)				(7,162)				(5,047)	(7,162)
Federal net operating loss	0				155,839				0	155,839
Loss reserves	3,113				4,994				3,113	4,994
Unrealized depreciation (appreciation) in investments	9,407				(7,782)				9,407	(7,782)
Mortgage investments	8,307				8,963				8,307	8,963
Deferred compensation	8,662				7,265				8,662	7,265
AMT credit carryforward	17,521				37,017				17,521	37,017
Other, net	(4,587)				6,051				(4,587)	6,051
Components of provisions for (benefit from) income taxes [Abstract]
Current Federal									(16,272)	73,348	$ 9,470
Deferred Federal									185,598	351,677	160,657
Other									4,727	3,710	2,070
Provision for income taxes	36,963	$ 49,994	$ 50,708	$ 36,388	218,142	$ 64,440	$ 61,994	$ 84,159	174,053	428,735	172,197
Income tax expense due to remeasurement of net deferred tax assets									133,000
Income taxes paid									$ 12,200	$ 22,000	$ 4,500
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)									21.00%	35.00%	35.00%
Additional income tax provision related to the rate decrease included in the Tax Act									0.00%	17.00%	0.00%
Additional income tax provision related to IRS litigation (in hundredths)									(0.30%)	3.70%	0.10%
Tax exempt municipal bond interest (in hundredths)									(0.70%)	(1.40%)	(1.90%)
Other, net (in hundredths)									0.60%	0.40%	0.30%
Effective income tax provision rate (in hundredths)									20.60%	54.70%	33.50%
Unrecognized tax benefits [Roll Forward]
Balance at beginning of year				$ 142,821				$ 108,245	$ 142,821	$ 108,245	$ 107,120
Additions for tax positions of prior years									0	35,003	1,125
Reductions for tax positions of prior years									(3,070)	(427)	0
Settlements									(139,751)	0	0
Balance at end of year	$ 0				142,821				0	142,821	$ 108,245
Interest expense recognized during period									(3,100)
Unrecognized tax benefits, accrued interest					$ 52,000					$ 52,000
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