Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2007	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets														$ 636,449	$ 791,286
Total deferred tax liabilities														(28,794)	(29,206)
Net deferred tax asset														607,655	762,080
Components of net deferred tax liability [Abstract]
Unearned premium reserves														40,153	33,262
Benefit plans														(12,350)	(14,283)
Federal net operating loss														520,812	680,975
Loss reserves														10,883	15,536
Unrealized depreciation in investments														11,211	8,904
Mortgage investments														17,751	17,386
Deferred compensation														12,517	12,927
Other, net														6,678	7,373
Net deferred tax asset														607,655	762,080
Tax provision (benefit) [Abstract]
Provision for income taxes before valuation allowance										$ 172,197	$ 163,497	$ 91,607
Change in valuation allowance										0	(161,158)	(88,833)
Reversal of the valuation allowance										0	(686,652)	0
Provision for (benefit from) income taxes		$ 54,551	$ 27,131	$ 56,018	$ 34,497	$ 6,584	$ (695,604)	$ 1,322	$ 3,385	172,197	(684,313)	2,774
Valuation allowance															0	$ 902,300
Change in deferred tax valuation allowance, included in other comprehensive income											(54,500)	(13,100)
Net operating loss carryforwards, regular tax basis														1,489,000
Net operating loss carryforwards for computing the alternative minimum tax														589,000
Components of provisions for (benefit from) income taxes [Abstract]
Current Federal										9,470	8,067	2,391
Deferred Federal										160,657	(686,652)	1
Other										2,070	(5,728)	382
Provision for (benefit from) income taxes		54,551	$ 27,131	$ 56,018	34,497	6,584	$ (695,604)	$ 1,322	3,385	172,197	(684,313)	2,774
Income taxes paid										$ 4,500	$ 5,400	$ 1,300
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax rate (in hundredths)										35.00%	35.00%	35.00%
Valuation allowance (in hundredths)										0.00%	(173.80%)	(34.90%)
Tax exempt municipal bond interest (in hundredths)										(1.90%)	(0.80%)	(0.40%)
Other, net (in hundredths)										0.40%	(0.70%)	1.40%
Effective income tax provision (benefit) rate (in hundredths)										33.50%	(140.30%)	1.10%
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														200,600
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														50,700
Unrecognized tax benefits	$ 108,245	108,245			107,120	107,120			106,230	$ 107,120	$ 106,230	$ 105,366		108,245	107,120	$ 106,230
Unrecognized tax benefits [Roll Forward]
Balance at beginning of year	108,245				$ 107,120				$ 106,230	107,120	106,230	105,366
Additions based on tax positions related to the current year										0	0	0
Additions for tax positions of prior years										1,125	890	864
Reductions for tax positions of prior years										0	0	0
Settlements										0	0	0
Balance at end of year		$ 108,245				$ 107,120				108,245	107,120	$ 106,230
Unrecognized tax benefits [Abstract]
Total amount of unrecognized tax benefits that would affect effective tax rate														94,600
Interest expense recognized during period										$ 1,100
Unrecognized tax benefits, accrued interest														28,900	$ 27,800
Significant change in unrecognized tax benefits														108,200
Approximate net cash outflows upon resolution of IRS matters														$ 52,000
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
Forecast | Internal Revenue Service (IRS) | Minimum
Information regarding income tax examinations [Abstract]
Income Tax Examination, Estimate of Possible Loss	15,000
Forecast | Internal Revenue Service (IRS) | Maximum
Information regarding income tax examinations [Abstract]
Income Tax Examination, Estimate of Possible Loss	$ 25,000