Segment Information - Schedule of Combined Segment Adjusted EBITDA to Net Income Attributable to Arconic (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total segment operating profit	$ 389	$ 377	$ 1,221	$ 1,282
Restructuring and other charges	2	(19)	(20)	(118)
Operating income	345	310	1,002	913
Interest expense	(88)	(100)	(291)	(398)
Other (expense) income, net	(8)	(38)	(69)	410
Income before income taxes	249	172	642	925
Segment Reconciling Items
Segment Reporting Information [Line Items]
Restructuring and other charges	2	(19)	(20)	(118)
Corporate expense	(46)	(48)	(199)	(251)
Operating income	345	310	1,002	913
Interest expense	(88)	(100)	(291)	(398)
Other (expense) income, net	(8)	(38)	(69)	410
Income before income taxes	$ 249	$ 172	$ 642	$ 925