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Equity Investments - Summary of Unaudited Financial Information for Alcoa's Equity Investments (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Investments Schedule [Abstract]    
Sales $ 892 $ 886
Cost of goods sold 681 695
Income before interest expense and income taxes 142 87
Net income (loss) 15 (41)
Net income (loss) attributable to significant equity investments $ 15 $ (41)