Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2019
Goodwill [Roll Forward]
Goodwill gross, beginning balance	$ 5,247	$ 5,282
Accumulated impairment losses	(747)	(747)
Goodwill, net	4,500	4,535
Acquisitions and Divestitures	(13)	(1)
Translation and other	6	(34)
Transfer from Engineered Structures to Aluminum Extrusions	0
Goodwill gross, ending balance	5,240	5,247
Accumulated impairment losses	(747)	(747)
Goodwill, ending balance	4,500	4,535	$ 4,493
Engineered Products and Forgings
Goodwill [Roll Forward]
Goodwill, net	4,186
Acquisitions and Divestitures	(13)
Goodwill, ending balance	4,186		4,067	$ 7
Global Rolled Products
Goodwill [Roll Forward]
Goodwill, net	314
Acquisitions and Divestitures	0
Goodwill, ending balance	314		426	$ 68
Latin America Extrusions (LAE)
Goodwill [Roll Forward]
Goodwill gross, ending balance	25
Total segment sales | Engineered Products and Forgings
Goodwill [Roll Forward]
Goodwill gross, beginning balance	4,905	4,931
Accumulated impairment losses	(719)	(719)
Goodwill, net	4,186	4,212
Acquisitions and Divestitures		(1)
Translation and other	4	(25)
Transfer from Engineered Structures to Aluminum Extrusions	(110)
Goodwill gross, ending balance	4,786	4,905
Accumulated impairment losses	(719)	(719)
Goodwill, ending balance	4,186	4,212	4,067
Total segment sales | Global Rolled Products
Goodwill [Roll Forward]
Goodwill gross, beginning balance	342	351
Accumulated impairment losses	(28)	(28)
Goodwill, net	314	323
Acquisitions and Divestitures		0
Translation and other	2	(9)
Transfer from Engineered Structures to Aluminum Extrusions	110
Goodwill gross, ending balance	454	342
Accumulated impairment losses	(28)	(28)
Goodwill, ending balance	$ 314	$ 323	$ 426