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Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	12 Months Ended
	Dec. 31, 2014
Structures [Member] | Alumina [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	31 years
Structures [Member] | Alumina [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	32 years
Structures [Member] | Primary Metals [Member] | Aluminum Smelting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	35 years
Structures [Member] | Primary Metals [Member] | Power Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	31 years
Structures [Member] | Global Rolled Products [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	28 years
Structures [Member] | Engineered Products and Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	27 years
Machinery and Equipment [Member] | Alumina [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	26 years
Machinery and Equipment [Member] | Alumina [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	15 years
Machinery and Equipment [Member] | Primary Metals [Member] | Aluminum Smelting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	22 years
Machinery and Equipment [Member] | Primary Metals [Member] | Power Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	22 years
Machinery and Equipment [Member] | Global Rolled Products [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	22 years
Machinery and Equipment [Member] | Engineered Products and Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	18 years

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

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