Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 161	$ 119	$ 424	$ 653
Less: Preferred stock dividends declared	(1)	(18)	(2)	(53)
Net income available to Arconic common shareholders - basic	160	101	422	600
Add: Interest expense related to convertible notes	3	2	8	7
Add: Dividends related to mandatory convertible preferred stock	0	0	0	50
Net income available to Arconic common shareholders - diluted	$ 163	$ 103	$ 430	$ 657
Average shares outstanding - basic (in shares)	483	442	483	441
Effect of dilutive securities:
Stock options (in shares)	0	1	1	2
Stock and performance awards (in shares)	5	5	5	5
Mandatory convertible preferred stock (in shares)	0	0	0	39
Convertible notes (in shares)	14	14	14	14
Weighted average shares outstanding - diluted (in shares)	502	462	503	501