Investments - Summary of Unaudited Financial Information for Alcoa's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investments Schedule [Abstract]
Sales	$ 888	$ 1,015	$ 2,684	$ 2,872
Cost of goods sold	840	796	2,200	2,353
Net income (loss)	$ (6)	$ 20	$ 23	$ (66)