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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2011 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Tax Credit Carryforward [Line Items] |  |  |  |
Tolling Payment Payable to AFG |  | $ 30,496 | $ 28,691 |
Impact of AMT Repeal on Current Taxes |  | $ 29,581 |  |
Congressional Budget Sequestration Haircut |  | 6.60% |  |
Impact of new Tax Law on Deferred Taxes |  | $ 31,418 |  |
Net Impact of New Tax Law |  | $ 1,886 |  |
AMT Installment Payment End Date |  | Dec. 31, 2021 |  |
Tolling Payment Payable to AFG |  | $ 31,133 | 28,691 |
Unrecognized Tax Benefits, Interest on Income Taxes Accrued |  | 0 | $ 0 | $ 0
NOL allocated amount | $ 3,650,000 | 3,694,844 |  |
AFG [Member] |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
NOL allocated amount |  | 1,412,639 |  |
Percentage Of Notional Federal Tax Liability | 50.00% |  |  |
U. S. Federal Net Operating Tax [Member] |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
NOL allocated amount |  | $ 3,694,844 |  |
Minimum [Member] |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating Loss Carryforwards, Expiration Date |  | Dec. 31, 2029 |  |
Maximum [Member] |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |
Operating Loss Carryforwards, Expiration Date |  | Dec. 31, 2032 |  |

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