Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments Schedule [Abstract]
Sales	$ 979	$ 888	$ 2,853	$ 2,684
Cost of goods sold	792	840	2,186	2,200
Net income (loss)	$ 55	$ (6)	$ 97	$ 23