Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity Method Investment Summarized Financial Information Income Statement [Abstract]
Sales	$ 1,051	$ 1,343	$ 3,474	$ 3,963
Cost of goods sold	809	1,062	2,789	3,100
Net income (loss)	$ 4	$ 46	$ (73)	$ 143

The amount of the cost of sales reported by an equity method investment of the entity.

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The amount of net income (loss) reported by an equity method investment of the entity.

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Amount of revenue from sale of product and rendering of service reported by equity method investee.

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