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Financial instruments disclosures - Additional information (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 12,405 $ 12,653
Expected unconditional guaranteed royalty payments to be received 155 113
Expected unconditional guaranteed royalty payments to be received, unadjusted $ 56  
Percentage of change in coal spot price 15.00%  
Fair value hedges | Currency risk    
Disclosure of detailed information about financial instruments [line items]    
Hedged item, assets $ 273 388
Hedged item, liabilities 162 140
Interest rate swap contract    
Disclosure of detailed information about financial instruments [line items]    
Nominal value 1,500  
Level 1    
Disclosure of detailed information about financial instruments [line items]    
Bonds, carrying value 7,400 7,600
Bonds, fair value 9,000 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 $ 149 198
Decrease in carrying value of forward contract due to decrease in spot price (41) (46)
Level 3 | Aluminium Forward Contracts Embedded In Electricity Purchase Contracts    
Disclosure of detailed information about financial instruments [line items]    
Long-term embedded derivatives at fair value (92) 126
Level 3 | Fair value    
Disclosure of detailed information about financial instruments [line items]    
Borrowings $ 4,700 $ 4,700