Segment Information - Schedule of Combined Segment Adjusted EBITDA to Net Income Attributable to Arconic (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total segment operating profit	$ 524	$ 391	$ 1,511	$ 1,225
Restructuring and other charges	(119)	2	(630)	(20)
Operating income	326	345	619	1,002
Interest expense	(86)	(88)	(256)	(291)
Other expense, net	(31)	(8)	(92)	(69)	$ (92)	$ (69)
Income before income taxes	209	249	271	642
Segment Reconciling Items
Segment Reporting Information [Line Items]
Restructuring and other charges	(119)	2	(630)	(20)
Corporate expense	(79)	(48)	(262)	(203)
Operating income	326	345	619	1,002
Interest expense	(86)	(88)	(256)	(291)
Other expense, net	(31)	(8)	(92)	(69)
Income before income taxes	$ 209	$ 249	$ 271	$ 642