Restructuring and Other Charges, Net - Schedule of Restructuring and Other Charges by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	$ 7	$ 2
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	15	4
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net (D)	(8)	(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)	$ 15	$ 2