Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments Schedule [Abstract]
Sales	$ 1,343	$ 979	$ 3,963	$ 2,853
Cost of goods sold	1,062	792	3,100	2,186
Net income	$ 46	$ 55	$ 143	$ 97

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- Definition

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

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- Definition

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

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- Definition

The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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