Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income attributable to Alcoa	$ 44	$ 149	$ 379	$ 109
Less: preferred stock dividends declared	18		52	1
Net income available to Alcoa common shareholders - basic	26	149	327	108
Add: dividends related to mandatory convertible preferred stock	0	0	0	0
Add: interest expense related to convertible notes	0	0	0	0
Net income available to Alcoa common shareholders - diluted	$ 26	$ 149	$ 327	$ 108
Average shares outstanding - basic	1,281	1,177	1,241	1,150
Stock options	1	8	4	6
Stock and performance awards	12	12	12	11
Mandatory convertible preferred stock		8		3
Convertible notes	0	0	0	0
Average shares outstanding - diluted	1,294	1,205	1,257	1,170