Earnings Per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net (loss) income attributable to Alcoa	$ (701)	$ 44	$ 140	$ 195	$ 159	$ 149	$ 138	$ (178)	$ (322)	$ 268	$ (2,285)
Less: preferred stock dividends declared									69	21	2
Net (loss) income available to Alcoa common shareholders-basic									(391)	247	(2,287)
Add: dividends related to mandatory convertible preferred stock									0	0	0
Add: interest expense related to convertible notes									0	0	0
Net (loss) income available to Alcoa common shareholders-diluted									$ (391)	$ 247	$ (2,287)
Average shares outstanding-basic									1,259	1,162	1,070
Stock options										7
Stock and performance awards										11
Mandatory convertible preferred stock									0	0	0
Convertible notes									0	0	0
Average shares outstanding-diluted									1,259	1,180	1,070