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Financial instruments and risk management - Summary of changes in market price relevant to alumina purchase price swaps outstanding (Detail) - Commodity price risk [Member] - Aluminium [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Effect on net earnings [Member] |  |
Disclosure Of Financial Instruments [Line Items] |  |
Percentage increase in market price change | 10.00% |
Percentage decrease in market price change | (10.00%) |
Decrease in market price change of commodities | $ (102) | $ (203)
Increase in market price change of commodities | $ 35 | $ 212
Effect on equity [Member] |  |
Disclosure Of Financial Instruments [Line Items] |  |
Percentage increase in market price change | 10.00% |
Percentage decrease in market price change | (10.00%) |
Decrease in market price change of commodities | $ (101) |
Increase in market price change of commodities | $ 103 |

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