Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	Dec. 31, 2023
Software [Member] | Alumina [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	3 years
Software [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	3 years
Other Intangible Assets [Member] | Alumina [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	25 years
Other Intangible Assets [Member] | Aluminum Smelting and Casting [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	40 years
Other Intangible Assets [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	29 years