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Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension and Postretirement Plans' Assets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items] |
Net payable which represents securities purchased not yet settled less interest and dividends earned on various investments | $ 20
Level 1 [Member] | Equity Securities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Common stock related to plan contribution | $ 36

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- Definition Common stock related to plan contribution. + References No definition available. + Details
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Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Net payable which represents securities purchased not yet settled less interest and dividends earned on various investments. + References No definition available. + Details
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Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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