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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 185	$ 177	$ 668	$ 389
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	167	4	628	90
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	18	173	40	299
Bauxite [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	5	1	5	1
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	15	1	16	3
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 147	$ 2	$ 607	$ 86