Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income attributable to Alcoa	$ 135	$ 140	$ 151	$ 335
Less: preferred stock dividends declared	17	17	35	35
Net income available to Alcoa common shareholders - basic	118	123	116	300
Add: interest expense related to convertible notes	2
Add: dividends related to mandatory convertible preferred stock	0	0	0	0
Net income available to Alcoa common shareholders - diluted	$ 120	$ 123	$ 116	$ 300
Average shares outstanding - basic	1,315	1,222	1,314	1,222
Stock options	2	4	1	5
Stock and performance awards	11	11	11	11
Convertible notes	28
Mandatory convertible preferred stock	0	0	0	0
Average shares outstanding - diluted	1,356	1,237	1,326	1,238