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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 |  |  |

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- Definition Percentage of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations, attributable to increase (decrease) in income tax rate pursuant to Tax Cuts and Jobs Act of 2017. + References No definition available. + Details
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