Segment Information - Schedule of Combined Segment Adjusted EBITDA to Net Income Attributable to Arconic (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total segment operating profit	$ 538	$ 432	$ 985	$ 832
Restructuring and other charges	(499)	(15)	(511)	(22)
Operating (loss) income	(81)	324	293	657
Interest expense	(85)	(89)	(170)	(203)
Other expense, net	(29)	(41)	(61)	(61)	$ (61)	$ (61)
(Loss) income before income taxes	(195)	194	62	393
Segment Reconciling Items
Segment Reporting Information [Line Items]
Restructuring and other charges	(499)	(15)	(511)	(22)
Corporate expense	(120)	(93)	(181)	(153)
Operating (loss) income	(81)	324	293	657
Interest expense	(85)	(89)	(170)	(203)
Other expense, net	(29)	(41)	(61)	(61)
(Loss) income before income taxes	$ (195)	$ 194	$ 62	$ 393