Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation Of Revenue [Line Items]
Sales	$ 2,904	$ 2,602	$ 8,409	$ 7,956
Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,836	1,666	5,431	5,336
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	996	840	2,790	2,328
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	92	107	249	343
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	37	32	99	86
Other [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (57)	$ (43)	$ (160)	$ (137)