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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, 2021
USD ($)
t
Dec. 31, 2020
USD ($)
t
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 573,653 640,963
Notional amount | $ $ 1,377 $ 1,522
Average hedged rate (in US$ per tonne) 2,401 2,375
Due within 1 year    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 72,555 72,548
Notional amount | $ $ 162 $ 159
Average hedged rate (in US$ per tonne) 2,234 2,189
Between 1 and 5 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 289,867 287,587
Notional amount | $ $ 683 $ 663
Average hedged rate (in US$ per tonne) 2,355 2,305
Between 5 and 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 211,231 280,828
Notional amount | $ $ 532 $ 700
Average hedged rate (in US$ per tonne) 2,520 2,495
After 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 0 0
Notional amount | $ $ 0 $ 0
Average hedged rate (in US$ per tonne) 0 0