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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income (loss) attributable to Alcoa common shareholders	$ 140	$ 138	$ 335	$ (40)
Less: preferred stock dividends declared	17		35	1
Net income (loss) available to Alcoa common shareholders - basic	123	138	300	(41)
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 (loss) available to Alcoa common shareholders - diluted	$ 123	$ 138	$ 300	$ (41)
Average shares outstanding - basic	1,222	1,173	1,222	1,137
Stock options	4	6	5
Stock and performance awards	11	10	11
Mandatory convertible preferred stock	0	0	0	0
Convertible notes	0	0	0	0
Average shares outstanding - diluted	1,237	1,189	1,238	1,137