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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, 2023
USD ($)
uSDollarPerTonne
t
Dec. 31, 2022
USD ($)
t
uSDollarPerTonne
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 428,686 501,498
Nominal | $ $ 1,050 $ 1,216
Average hedged rate (in US$ per tonne) | uSDollarPerTonne 2,449 2,425
Due within 1 year    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 72,617 72,812
Nominal | $ $ 169 $ 166
Average hedged rate (in US$ per tonne) | uSDollarPerTonne 2,331 2,282
Between 1 and 5 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 289,801 289,868
Nominal | $ $ 711 $ 697
Average hedged rate (in US$ per tonne) | uSDollarPerTonne 2,452 2,404
Between 5 and 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | t 66,268 138,818
Nominal | $ $ 170 $ 353
Average hedged rate (in US$ per tonne) | uSDollarPerTonne 2,564 2,542