Goodwill and Other Intangible Assets - Summary of Goodwill which is Included in Other Noncurret Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 145	$ 150
Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	145	150
Bauxite [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	2	2
Alumina [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	2	3
Corporate Segment [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	$ 141	$ 145