Goodwill and Other Intangible Assets - Summary of Goodwill which is Included in Other Noncurret Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill	$ 989	$ 989
Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	154	155
Bauxite [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	2	2
Alumina [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	4	4
Corporate Segment [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	$ 148	$ 149