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Financial instruments and risk management - Summary of notional aluminum forward sales contracts embedded in power contracts (Detail) $ in Millions | Dec. 31, 2024 USD ($) t uSDollarPerTonne | Dec. 31, 2023 USD ($) t uSDollarPerTonne
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Disclosure of detailed information about financial instruments [line items] |  |
Nominal amount (tonnes) | t | 359,572 | 428,686
Nominal amount (US$ millions) | $ | $ 890 | $ 1,050
Average hedged rate (US$ per tonne) | uSDollarPerTonne | 2,474 | 2,449
Due within 1 year |  |
Disclosure of detailed information about financial instruments [line items] |  |
Nominal amount (tonnes) | t | 73,117 | 72,617
Nominal amount (US$ millions) | $ | $ 174 | $ 169
Average hedged rate (US$ per tonne) | uSDollarPerTonne | 2,382 | 2,331
Between 1 and 5 years |  |
Disclosure of detailed information about financial instruments [line items] |  |
Nominal amount (tonnes) | t | 286,455 | 289,801
Nominal amount (US$ millions) | $ | $ 716 | $ 711
Average hedged rate (US$ per tonne) | uSDollarPerTonne | 2,498 | 2,452
Between 5 and 10 years |  |
Disclosure of detailed information about financial instruments [line items] |  |
Nominal amount (tonnes) | t | 0 | 66,268
Nominal amount (US$ millions) | $ | $ 0 | $ 170
Average hedged rate (US$ per tonne) | uSDollarPerTonne | 0 | 2,564

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