Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Sales	$ 3,109	$ 2,365	$ 8,812	$ 6,894
Primary Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	2,255	1,311	6,094	3,810
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	752	697	2,196	2,004
Flat-Rolled Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales		284	320	827
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	117	26	205	100
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	52	50	138	170
Other Products [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (67)	$ (3)	$ (141)	$ (17)