Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2014
Software [Member] | Alumina [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	4 years
Software [Member] | Primary Metals [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	7 years
Software [Member] | Global Rolled Products [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	9 years
Software [Member] | Engineered Products and Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	11 years
Other Intangible Assets [Member] | Alumina [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	35 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	12 years
Other Intangible Assets [Member] | Engineered Products and Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	24 years