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Derivatives and Other Financial Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities and Gross Amounts Offset in Accompanying Consolidated Balance Sheet (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012
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Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Margin on Derivative held from counterparties | $ 9

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- Definition Derivative Margin Held From Counterparty + References No definition available. + Details
Name: | aa_DerivativeMarginHeldFromCounterparty
Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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