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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	$ 33	$ 37	$ 40	$ 39
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	3	37	18	41
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	30		22	(2)
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)				2
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	$ 3	$ 37	$ 18	$ 39