Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2015
Software [Member] | Alumina [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	7 years
Software [Member] | Primary Metals [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 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	7 years
Software [Member] | Transportation and Construction Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	8 years
Other Intangible Assets [Member] | Alumina [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	15 years
Other Intangible Assets [Member] | Primary Metals [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	37 years
Other Intangible Assets [Member] | Global Rolled Products [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	14 years
Other Intangible Assets [Member] | Engineered Products and Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	32 years
Other Intangible Assets [Member] | Transportation and Construction Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	23 years