Segment Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Sales	$ 2,148	$ 2,711	$ 4,529	$ 5,430
Primary Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,202	1,382	2,499	2,776
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	600	862	1,307	1,759
Flat-Rolled Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	271	327	543	639
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	61	63	120	121
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	22	85	74	154
Other Products [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (8)	$ (8)	$ (14)	$ (19)