Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to Alcoa	$ 16	$ 195
Less: preferred stock dividends declared	18	17
Net (loss) income available to Alcoa common shareholders - basic	(2)	178
Add: dividends related to mandatory convertible preferred stock	0	0
Add: interest expense related to convertible notes	0	0
Net (loss) income available to Alcoa common shareholders-diluted	$ (2)	$ 178
Average shares outstanding - basic	1,314	1,221
Stock options		6
Stock and performance awards		11
Mandatory convertible preferred stock	0	0
Convertible notes	0	0
Average shares outstanding - diluted	1,314	1,238