Restructuring and Other Charges, Net - Schedule of Restructuring and Other Charges, Net by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	$ 14	$ 18	$ 19	$ 220
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	6	8	9	205
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	8	10	10	15
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	0	8	0	205
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	$ 6	$ 0	$ 9	$ 0