Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2007	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets													$ 258,663	$ 636,449
Total deferred tax liabilities													(24,282)	(28,794)
Net deferred tax asset													234,381	607,655
Components of net deferred tax asset [Abstract]
Unearned premium reserves													29,196	40,153
Benefit plans													(7,162)	(12,350)
Federal net operating loss													155,839	520,812
Loss reserves													4,994	10,883
Unrealized (appreciation) depreciation in investments													(7,782)	11,211
Mortgage investments													8,963	17,751
Deferred compensation													7,265	12,517
AMT credit carryforward													37,017	2,215
Other, net													6,051	4,463
Tax provision (benefit) [Abstract]
Provision for income taxes before valuation allowance											$ 163,497
Change in valuation allowance											(161,158)
Reversal of the valuation allowance											(686,652)
Provision for (benefit from) income taxes	$ 218,142	$ 64,440	$ 61,994	$ 84,159	$ 54,551	$ 27,131	$ 56,018	$ 34,497	$ 428,735	$ 172,197	(684,313)
Change in deferred tax valuation allowance, included in other comprehensive income											(54,500)
Net operating loss carryforwards, regular tax basis													742,000
Components of provisions for (benefit from) income taxes [Abstract]
Current Federal									73,348	9,470	8,067
Deferred Federal									351,677	160,657	(686,652)
Other									3,710	2,070	(5,728)
Provision for (benefit from) income taxes	218,142	$ 64,440	$ 61,994	84,159	54,551	$ 27,131	$ 56,018	34,497	428,735	172,197	(684,313)
Income tax expense due to remeasurement of net deferred tax assets									133,000
Income taxes paid									$ 22,000	$ 4,500	$ 5,400
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)									35.00%	35.00%	35.00%
Additional income tax provision related to the rate decrease included in the Tax Act									17.00%	0.00%	0.00%
Additional income tax provision related to IRS litigation (in hundredths)									3.70%	0.10%	0.10%
Valuation allowance (in hundredths)									0.00%	0.00%	(173.80%)
Tax exempt municipal bond interest (in hundredths)									(1.40%)	(1.90%)	(0.80%)
Other, net (in hundredths)									0.40%	0.30%	(0.80%)
Effective income tax provision (benefit) rate (in hundredths)									54.70%	33.50%	(140.30%)
Information regarding income tax examinations [Abstract]
Amount of IRS assessment for unpaid taxes and penalties related to REMIC issue													197,500
Estimate of federal interest that may be due													205,000
Amount of payment made related to the IRS assessment on the REMIC issue												$ 65,200
Amount of IRS assessment for unpaid taxes and penalties related to disallowance of carryback of 2009 net operating loss													261,400
Estimate of additional state income taxes and interest that may be due													85,800
Unrecognized tax benefits	142,821			108,245	108,245			107,120	$ 108,245	$ 107,120	$ 106,230		142,821	108,245
Provision for additional taxes and interest									29,000
Unrecognized tax benefits [Roll Forward]
Balance at beginning of year				$ 108,245				$ 107,120	108,245	107,120	106,230
Additions based on tax positions related to the current year									0	0	0
Additions for tax positions of prior years									35,003	1,125	890
Reductions for tax positions of prior years									(427)	0	0
Settlements									0	0	0
Balance at end of year	$ 142,821				$ 108,245				142,821	$ 108,245	107,120
Unrecognized tax benefits [Abstract]
Total amount of unrecognized tax benefits that would affect effective tax rate													129,300
Interest expense recognized during period									$ 23,100
Unrecognized tax benefits, accrued interest													52,000	$ 28,900
Significant change in unrecognized tax benefits													142,800
Approximate net cash outflows upon resolution of IRS matters													$ 55,000
Deferred Tax Asset, Changes In Current Period
Tax provision (benefit) [Abstract]
Change in valuation allowance											154,800
Deferred Tax Asset, Changes In Current Period | Other Comprehensive Income (Loss)
Tax provision (benefit) [Abstract]
Change in valuation allowance											(6,300)
Deferred Tax Asset, Changes In Current Period | Income Tax Expense (Benefit)
Tax provision (benefit) [Abstract]
Change in valuation allowance											161,100
Deferred Tax Asset, Changes In Future Periods
Tax provision (benefit) [Abstract]
Change in valuation allowance											747,500
Deferred Tax Asset, Changes In Future Periods | Other Comprehensive Income (Loss)
Tax provision (benefit) [Abstract]
Change in valuation allowance											60,800
Deferred Tax Asset, Changes In Future Periods | Income Tax Expense (Benefit)
Tax provision (benefit) [Abstract]
Change in valuation allowance											$ 686,700