Restructuring and Other Charges - Schedule of Restructuring and Other Charges by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 138	$ 297	$ 527	$ 309	$ 318
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			215	56	142
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			312	253	176
Bauxite [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			1	2	(5)
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			112	3	72
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 102	$ 51	$ 75