Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Disaggregation Of Revenue [Line Items]
Sales	$ 3,369	$ 2,599
Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,955	1,661
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,211	890
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	242	61
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	33	33
Other [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (72)	$ (46)