Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	Dec. 31, 2023
Structures [Member] | Alumina [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
Machinery and Equipment [Member] | Alumina [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	27 years
Machinery and Equipment [Member] | Aluminum Smelting and Casting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	22 years
Machinery and Equipment [Member] | Energy Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	25 years