Goodwill and Other Intangible Assets - Summary of Goodwill which is Included in Other Noncurret Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Goodwill [Line Items]
Goodwill	$ 142	$ 146
Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	142	146
Alumina [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	3	4
Aluminum [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	0
Corporate Segment [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	$ 139	$ 142