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Financial instruments and risk management - Fair values - Additional information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 13,531 $ 13,831
Expected unconditional royalty payments to be received $ 136 113
Percentage of change in spot price 15.00%  
Forecast    
Disclosure of detailed information about financial instruments [line items]    
Expected unconditional royalty payments to be received $ 53  
Level 1    
Disclosure of detailed information about financial instruments [line items]    
Borrowings at fair value 7,300 7,600
Bonds, fair value 8,700 9,500
Level 3    
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 4,200 4,200
Percentage of change in coal spot price 15.00%  
Increase in carrying value of forward contract due to coal spot price assumptions $ 63 198
Decrease in carrying value of forward contract due to decrease in spot price 53 46
Level 3 | Aluminium embedded derivatives separated from the power contract    
Disclosure of detailed information about financial instruments [line items]    
Long-term embedded derivatives with fair value 146 126
Level 3 | Fair value    
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 4,400 $ 4,700