Segment and Geographic Area Information - Sales by Major Product Grouping (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
Product Information [Line Items]
Sales revenue, goods, net	$ 3,271	$ 3,236	$ 3,261	$ 3,192	$ 2,967	$ 3,138	$ 3,234	$ 3,055	$ 12,960	$ 12,394	$ 12,413
Flat-rolled aluminum
Product Information [Line Items]
Sales revenue, goods, net									4,992	4,864	5,253
Fastening systems and rings
Product Information [Line Items]
Sales revenue, goods, net									2,102	2,060	2,168
Investment castings
Product Information [Line Items]
Sales revenue, goods, net									1,983	1,870	1,812
Other extruded and forged products
Product Information [Line Items]
Sales revenue, goods, net									1,565	1,495	1,332
Architectural aluminum systems
Product Information [Line Items]
Sales revenue, goods, net									1,065	1,010	951
Aluminum wheels
Product Information [Line Items]
Sales revenue, goods, net									805	689	790
Other
Product Information [Line Items]
Sales revenue, goods, net									$ 448	$ 406	$ 107