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Financial instruments and risk management - Fair Values Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 13,831 $ 14,115
Expected unconditional royalty payments to be received $ 113 124
Percentage Of Change In Spot Price 15.00%  
Level 1    
Disclosure of detailed information about financial instruments [line items]    
Borrowings at fair value $ 7,600 7,700
Bonds, fair value 9,500 9,100
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 $ 198 214
Decrease in carrying value of forward contract due to decrease in spot price 46 (57)
Level 3 | Fair value    
Disclosure of detailed information about financial instruments [line items]    
Borrowings 4,700 4,700
Aluminium embedded derivatives separated from the power contract | Level 3    
Disclosure of detailed information about financial instruments [line items]    
Long-term embedded derivatives with fair value $ 126 $ 120