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Financial instruments and risk management - Summary of impact of hedging instrument on statement of financial position (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]    
Nominal $ 1,522 $ 1,656
Carrying amount 134 58
Aluminium embedded derivatives separated from the power contract    
Disclosure of detailed information about financial instruments [line items]    
Nominal 1,522 1,656
Carrying amount 46 42
Change in fair value in the period 23 29
Embedded derivatives in other financial assets 66 66
Embedded derivatives in other financial liabilities 20 24
Highly Probable Forecast Aluminum Sales    
Disclosure of detailed information about financial instruments [line items]    
Cash flow hedge reserve 184 196
Change in fair value in the period (49) (50)
Highly Probable Forecast Aluminum Sales | Cash flow hedge reserve    
Disclosure of detailed information about financial instruments [line items]    
Total hedging gain/(loss) recognised reserves 27 36
Hedge ineffectiveness in the period (4) (7)
Amount reclassified from reserves to income statement $ 40 $ 19