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Schedule II - Income Taxes Schedule II Income Taxes (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended | 24 Months Ended | 36 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2011 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2017
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards | $ 3,650,000 | $ 3,694,844 |  |  |  | $ 3,694,844
Tolling Payment Payable to AFG |  | 30,496 | $ 28,691 |  | $ 28,691 | 30,496
TierABCCredit |  | 5,000 |  | $ 5,000 |  |
Domestic Tax Authority [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards |  | 3,694,844 |  |  |  | 3,694,844
Ambac Assurance [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards |  | 2,282,205 |  |  |  | 2,282,205
Percentage Of Notional Federal Tax Liability | 25.00% |  |  |  |  |
AACNetTaxableIncome |  |  |  |  |  | 1,367,795
AACNetTaxableIncomeFromTierA |  |  | $ 479,000 |  |  |
AACNetTaxableIncomeFromTierB |  |  |  |  | $ 607,124 | 888,795
AFG [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards |  | 1,412,639 |  |  |  | 1,412,639
Percentage Of Notional Federal Tax Liability | 50.00% |  |  |  |  |
Tier B [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards | $ 1,057,000 | $ 1,057,000 |  |  |  | $ 1,057,000
Maximum [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Operating Loss Carryforwards, Expiration Date |  | Dec. 31, 2032 |  |  |  |

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