Restructuring and Other Charges, Net - Schedule of Restructuring and Other Charges by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 37	$ 370	$ 39	$ 483
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	37	352	41	461
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		18	(2)	22
Bauxite [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		(1)
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			2	1
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 37	$ 353	$ 39	$ 460