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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2015
Compensation and Retirement Disclosure [Abstract] |  |
Settlements excluded from restructuring and other charges | $ 2 | $ 2

X
- Definition Defined benefit plan, recognized net gain (loss) due to settlements excluded from restructuring and other charges. + References No definition available. + Details
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Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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