Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Investments Schedule [Abstract]
Sales	$ 1,263	$ 1,253
Cost of goods sold	1,045	960
Net (loss) income	$ (48)	$ 65

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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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