Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	12 Months Ended
	Dec. 31, 2020
Structures [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	34 years
Structures [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	29 years
Structures [Member] | Aluminum Smelting and Casting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	37 years
Structures [Member] | Energy Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	33 years
Structures [Member] | Aluminum Rolling [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	32 years
Machinery and Equipment [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	16 years
Machinery and Equipment [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	29 years
Machinery and Equipment [Member] | Aluminum Smelting and Casting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	23 years
Machinery and Equipment [Member] | Energy Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	24 years
Machinery and Equipment [Member] | Aluminum Rolling [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	23 years