Segment and Geographic Information - Dchedule of Geo (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Sales Information [Line Items]
Sales	$ 3,174	$ 2,964	$ 2,859	$ 2,655	$ 2,537	$ 2,329	$ 2,323	$ 2,129	$ 11,652	$ 9,318	$ 11,199
Intersegment Eliminations [Member]
Sales Information [Line Items]
Sales									(2,619)	(2,100)	(2,978)
Primary Aluminum [Member]
Sales Information [Line Items]
Sales									6,168	5,204	6,214
Alumina [Member]
Sales Information [Line Items]
Sales									3,121	2,280	3,325
Flat-Rolled Aluminum [Member]
Sales Information [Line Items]
Sales									1,666	1,068	989
Energy [Member]
Sales Information [Line Items]
Sales									446	422	588
Bauxite [Member]
Sales Information [Line Items]
Sales									333	315	71
Other Products [Member]
Sales Information [Line Items]
Sales										$ 29	$ 12
Other Products [Member] | Intersegment Eliminations [Member]
Sales Information [Line Items]
Sales									$ (82)