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Investments - Summary of Unaudited Financial Information for Alcoa's Equity Investments (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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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)

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