Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Sales	$ 2,833	$ 2,148	$ 5,703	$ 4,529
Primary Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	2,112	1,202	3,839	2,499
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	684	600	1,444	1,307
Flat-Rolled Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales		271	320	543
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	34	61	86	120
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	49	22	88	74
Other Products [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (46)	$ (8)	$ (74)	$ (14)