Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Software [Member] | Bauxite Mining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	9 years
Software [Member] | Alumina Refining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 years
Software [Member] | Aluminum Smelting and Casting [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	5 years
Software [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	3 years
Software [Member] | Aluminum Rolling [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	4 years
Other Intangible Assets [Member] | Bauxite Mining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	10 years
Other Intangible Assets [Member] | Alumina Refining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	20 years
Other Intangible Assets [Member] | Aluminum Smelting and Casting [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	36 years
Other Intangible Assets [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	29 years
Other Intangible Assets [Member] | Aluminum Rolling [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	20 years