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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
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