Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Sales	$ 2,870	$ 2,381
Primary Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,727	1,297
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	760	707
Flat-Rolled Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	320	272
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	52	59
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	39	52
Other Products [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (28)	$ (6)