Segment and Geographic Area Information - Sales by Major Product Grouping (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Product Information [Line Items]
Sales	$ 14,014	$ 12,960	$ 12,394
Innovative flat-rolled products
Product Information [Line Items]
Sales	5,588	4,992	4,864
Engines
Product Information [Line Items]
Sales	2,940	2,708	2,560
Engineered structures
Product Information [Line Items]
Sales	1,839	1,743	1,683
Fastening systems
Product Information [Line Items]
Sales	1,531	1,484	1,463
Architectural aluminum systems
Product Information [Line Items]
Sales	1,140	1,065	1,010
Aluminum wheels
Product Information [Line Items]
Sales	969	805	689
Other
Product Information [Line Items]
Sales	$ 7	$ 163	$ 125