Restructuring and Other Charges, Net - Schedule of Restructuring and Other Charges by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	$ 33	$ 5	$ 73	$ 44
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	23	5	41	46
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	10		32	(2)
Bauxite [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)		1		1
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)		3		5
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	$ 23	$ 1	$ 41	$ 40