Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation Of Revenue [Line Items]
Sales	$ 3,018	$ 2,906	$ 6,387	$ 5,505
Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,993	1,934	3,948	3,595
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	830	904	2,041	1,794
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	203	96	445	157
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	38	29	71	62
Other [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (46)	$ (57)	$ (118)	$ (103)