Segment and Geographic Area Information - Sales by Major Product Grouping (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Product Information [Line Items]
Sales	$ 3,401	$ 3,559	$ 3,691	$ 3,541	$ 3,472	$ 3,524	$ 3,573	$ 3,445	$ 14,192	$ 14,014	$ 12,960
Innovative flat-rolled products
Product Information [Line Items]
Sales									5,471	5,604	5,000
Engine products
Product Information [Line Items]
Sales									3,452	3,220	2,965
Fastening systems
Product Information [Line Items]
Sales									1,561	1,531	1,484
Engineered structures
Product Information [Line Items]
Sales									1,123	1,081	1,023
Architectural aluminum systems
Product Information [Line Items]
Sales									1,118	1,135	1,069
Forged wheels
Product Information [Line Items]
Sales									969	966	828
Aluminum extrusions
Product Information [Line Items]
Sales									493	484	471
Other
Product Information [Line Items]
Sales									$ 5	$ (7)	$ 120