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Derivatives and Other Financial Instruments - Schedule of Carrying Values and Fair Values of Other Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Cash and Cash Equivalents [Member] |  |
Derivative [Line Items] |  |
Carrying value | $ 1,432 | $ 1,939
Fair value | 1,432 | 1,939
Restricted Cash [Member] |  |
Derivative [Line Items] |  |
Carrying value | 224 | 25
Fair value | 224 | 25
Noncurrent Receivables [Member] |  |
Derivative [Line Items] |  |
Carrying value | 21 | 30
Fair value | 21 | 30
Available-for-sale securities [Member] |  |
Derivative [Line Items] |  |
Carrying value | 89 | 92
Fair value | 89 | 92
Short-term Debt [Member] |  |
Derivative [Line Items] |  |
Carrying value | 591 | 62
Fair value | 591 | 62
Commercial Paper [Member] |  |
Derivative [Line Items] |  |
Carrying value | 43 | 224
Fair value | 43 | 224
Long Term Debt Due Within One Year [Member] |  |
Derivative [Line Items] |  |
Carrying value | 540 | 445
Fair value | 540 | 445
Long-Term Debt, Less Amount Due Within One Year [Member] |  |
Derivative [Line Items] |  |
Carrying value | 8,350 | 8,640
Fair value | $ 9,253 | $ 9,274

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- Definition Financial Instruments Owned At Carrying Value + References No definition available. + Details
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Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6475986&loc=d3e42546-110969 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-BRD -Paragraph 38, 40 -IssueDate 2006-05-01 -Chapter 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-BRD -Paragraph 80 -Subparagraph Exhibit 4-3 -IssueDate 2006-05-01 -Chapter 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_FinancialInstrumentsOwnedAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant