Revenue - Schedule of Revenue by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 3,390	$ 2,964	$ 10,059	$ 8,478
Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	(16)	(25)	(53)	(47)
Primary Aluminum [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,658	1,608	5,176	4,458
Alumina [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,098	710	3,079	2,187
Flat-rolled Aluminum [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	472	410	1,417	1,286
Energy [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	115	157	261	340
Bauxite [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 63	$ 104	$ 179	$ 254