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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018 | Dec. 31, 2016 | Sep. 30, 2011
Tax Credit Carryforward [Line Items] |  |  |  |  |
Deferred Tax Liabilities, Net | $ 40,130 | $ 33,659 |  |  |
Tolling Payment Payable to AFG | 44,381 | 30,496 |  |  |
Impact of AMT Repeal on Current Taxes |  | 29,581 |  |  |
Impact of new Tax Law on Deferred Taxes |  | 31,418 |  |  |
Net Impact of New Tax Law |  | 1,886 |  |  |
Tolling Payment Payable to AFG | 13,885 |  |  | $ 28,691 |
Unrecognized Tax Benefits, Interest on Income Taxes Accrued | 0 | $ 0 |  | $ 0 |
NOL allocated amount | 3,423,704 |  |  |  | $ 3,650,000
AFG [Member] |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
NOL allocated amount | $ 1,260,583 |  |  |  |
U. S. Federal Net Operating Tax [Member] |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
NOL allocated amount |  |  | $ 3,423,704 |  |
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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