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, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	$ (4)	$ 14	$ 14	$ 19
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	(3)	6	(3)	9
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	(1)	8	17	10
Alumina [Member] \| Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	0	0	0	0
Aluminum Segment [Member] \| Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net	$ (3)	$ 6	$ (3)	$ 9

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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