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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, 2019
USD ($)
Tonne
Dec. 31, 2018
USD ($)
Tonne
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | Tonne 704,370 767,111
Notional amount | $ $ 1,656 $ 1,786
Average hedged rate (in US$ per tonne) 2,351 2,328
Within 1 year [member]    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | Tonne 65,226 56,481
Notional amount | $ $ 138 $ 114
Average hedged rate (in US$ per tonne) 2,114 2,013
Between 1 and 5 years [member]    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | Tonne 286,617 286,666
Notional amount | $ $ 647 $ 634
Average hedged rate (in US$ per tonne) 2,257 2,210
Between 5 and 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | Tonne 352,527 358,416
Notional amount | $ $ 871 $ 870
Average hedged rate (in US$ per tonne) 2,471 2,426
After 10 years    
Disclosure of detailed information about financial instruments [line items]    
Notional amount (in tonnes) | Tonne 0 65,548
Notional amount | $ $ 0 $ 168
Average hedged rate (in US$ per tonne) 0 2,562