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Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net income (loss) | $ 350 | $ (150) | $ 569 | $ (11)
Ma'aden Joint Venture [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity in net income (loss) of affiliated companies, before reconciling adjustments | 26 | (19) | 37 | (22)
Other |  | (1) | (4) | (4)
Alcoa Corporation’s equity in net (loss) income of affiliated companies | 26 | (20) | 33 | (26)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Sales | 741 | 507 | 1,423 | 1,092
Cost of goods sold | 508 | 446 | 1,011 | 908
Net income (loss) | 102 | (75) | 147 | (88)
Mining [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity in net income (loss) of affiliated companies, before reconciling adjustments | 9 | 3 | 10 | 9
Other | (1) | 2 | (1) | (1)
Alcoa Corporation’s equity in net (loss) income of affiliated companies | 8 | 5 | 9 | 8
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Sales | 197 | 209 | 370 | 427
Cost of goods sold | 146 | 130 | 269 | 278
Net income (loss) | 25 | (5) | 20 | 4
Energy [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity in net income (loss) of affiliated companies, before reconciling adjustments | 11 | 7 | 21 | 18
Other |  | 1 |  | (1)
Alcoa Corporation’s equity in net (loss) income of affiliated companies | 11 | 8 | 21 | 17
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Sales | 60 | 47 | 115 | 106
Cost of goods sold | 32 | 24 | 57 | 50
Net income (loss) | 27 | 20 | 52 | 47
Other [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity in net income (loss) of affiliated companies, before reconciling adjustments | (2) | (3) | (3) | (7)
Other | 8 | 5 | 11 | 10
Alcoa Corporation’s equity in net (loss) income of affiliated companies | 6 | 2 | 8 | 3
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Sales | 96 | 85 | 191 | 158
Cost of goods sold | 78 | 75 | 171 | 142
Net income (loss) | $ (4) | $ (6) | $ (6) | $ (15)

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