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v2.4.0.6

Recently Adopted and Recently Issued Accounting - Additional Information (Detail) | Sep. 30, 2012
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Indefinite-lived Intangible Assets [Line Items] |
Fair value of an indefinite-lived intangible asset | 50.00%

X
- Definition Percentage Of Assets Fair Value Disclosure + References No definition available. + Details
Name: | aa_PercentageOfAssetsFairValueDisclosure
Namespace Prefix: | aa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration