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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, 2018 USD ($) Tonne
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Due within 1 year [member] |
Disclosure Of Financial Instruments [Line Items] |
Notional amount (in tonnes) | Tonne | 56,481
Notional amount | $ | $ 114
Average hedged rate (in US$ per tonne) | 2,013
Between 1 and 5 years [Member] |
Disclosure Of Financial Instruments [Line Items] |
Notional amount (in tonnes) | Tonne | 286,666
Notional amount | $ | $ 634
Average hedged rate (in US$ per tonne) | 2,210
Between 5 and 10 years [Member] |
Disclosure Of Financial Instruments [Line Items] |
Notional amount (in tonnes) | Tonne | 358,416
Notional amount | $ | $ 870
Average hedged rate (in US$ per tonne) | 2,426
After 10 years [Member] |
Disclosure Of Financial Instruments [Line Items] |
Notional amount (in tonnes) | Tonne | 65,548
Notional amount | $ | $ 168
Average hedged rate (in US$ per tonne) | 2,562

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