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Financial instruments and risk management - Summary of notional aluminium forward sales contracts embedded in power contracts (Detail) $ in Millions | Dec. 31, 2019 USD ($) Tonne | Dec. 31, 2018 USD ($) Tonne
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Disclosure of detailed information about financial instruments [line items] |  |
Notional amount (in tonnes) | Tonne | 704,370 | 767,111
Notional amount | $ | $ 1,656 | $ 1,786
Average hedged rate (in US$ per tonne) | 2,351 | 2,328
Within 1 year [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount (in tonnes) | Tonne | 65,226 | 56,481
Notional amount | $ | $ 138 | $ 114
Average hedged rate (in US$ per tonne) | 2,114 | 2,013
Between 1 and 5 years [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount (in tonnes) | Tonne | 286,617 | 286,666
Notional amount | $ | $ 647 | $ 634
Average hedged rate (in US$ per tonne) | 2,257 | 2,210
Between 5 and 10 years |  |
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount (in tonnes) | Tonne | 352,527 | 358,416
Notional amount | $ | $ 871 | $ 870
Average hedged rate (in US$ per tonne) | 2,471 | 2,426
After 10 years |  |
Disclosure of detailed information about financial instruments [line items] |  |
Notional amount (in tonnes) | Tonne | 0 | 65,548
Notional amount | $ | $ 0 | $ 168
Average hedged rate (in US$ per tonne) | 0 | 2,562

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