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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, 2021
Dec. 31, 2020
Disclosure of detailed information about financial instruments [line items]    
Nominal amount $ 1,377 $ 1,522
Carrying amount 62 134
Aluminium embedded derivatives separated from the power contract    
Disclosure of detailed information about financial instruments [line items]    
Nominal amount 1,377 1,522
Carrying amount (124) 46
Change in fair value in the period (201) 23
Embedded derivatives in other financial assets 0 66
Embedded derivatives in other financial liabilities 124 20
Highly probable forecast aluminum sales    
Disclosure of detailed information about financial instruments [line items]    
Cash flow hedge reserve (11) 184
Change in fair value in the period 300 (49)
Highly probable forecast aluminum sales | Cash flow hedge reserve    
Disclosure of detailed information about financial instruments [line items]    
Total hedging gain/(loss) recognised reserves (211) 27
Hedge ineffectiveness in the period 10 (4)
Amount reclassified from reserves to income statement $ 17 $ (40)