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Recently Adopted and Recently Issued Accounting Guidance - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended
--- | --- | ---
Aug. 31, 2015 | Sep. 30, 2015 | Sep. 30, 2014
Accounting Changes and Error Corrections [Abstract] |  |  |
Decrease in other noncurrent assets | $ (60) | $ 348 | $ (5)
Decrease in long term debt non current | $ (60) |  |

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- Definition Increase Decrease In Long Term Debt Noncurrent + References No definition available. + Details
Name: | aa_IncreaseDecreaseInLongTermDebtNoncurrent
Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingChangesAndErrorCorrectionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The increase (decrease) during the reporting period in other noncurrent operating assets not separately disclosed in the statement of cash flows. + References No definition available. + Details
Name: | us-gaap_IncreaseDecreaseInOtherNoncurrentAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration