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Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail) | 12 Months Ended
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Dec. 31, 2012
Software [Member] | Alumina [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 10 years
Software [Member] | Primary Metals [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 9 years
Software [Member] | Global Rolled Products [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 10 years
Software [Member] | Engineered Products and Solutions [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 9 years
Other Intangible Assets [Member] | Primary Metals [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 36 years
Other Intangible Assets [Member] | Global Rolled Products [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 13 years
Other Intangible Assets [Member] | Engineered Products and Solutions [Member] |
Finite-Lived Intangible Assets [Line Items] |
Weighted-average useful lives of other intangible assets | 19 years

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

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration