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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, 2020
USD ($)
t
Dec. 31, 2019
USD ($)
t
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 640,963 704,370
Notional amount | $ $ 1,522 $ 1,656
Average hedged rate (in US$ per tonne) 2,375 2,351
Due within 1 year    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 72,548 65,226
Notional amount | $ $ 159 $ 138
Average hedged rate (in US$ per tonne) 2,189 2,114
Between 1 and 5 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 287,587 286,617
Notional amount | $ $ 663 $ 647
Average hedged rate (in US$ per tonne) 2,305 2,257
Between 5 and 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 280,828 352,527
Notional amount | $ $ 701 $ 871
Average hedged rate (in US$ per tonne) 2,495 2,471
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