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Financial instruments - Additional information (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2022
Dec. 31, 2021
Jun. 30, 2021
Disclosure of detailed information about financial instruments [line items]      
Cash and cash equivalents $ 11,412 $ 12,807 $ 14,027
Provisionally priced receivables 1,155 1,114  
Expected unconditional guaranteed royalty payments to be received 146 136  
Expected unconditional guaranteed royalty payments to be received, unadjusted $ 55    
Percentage of change in coal spot price 15.00%    
Level 3      
Disclosure of detailed information about financial instruments [line items]      
Percentage of change in coal spot price 15.00%    
Increase in carrying value of forward contract due to coal spot price assumptions $ 15 63  
Decrease in carrying value of forward contract due to decrease in spot price (41) (53)  
Borrowings 4,100 4,200  
Level 3 | Fair value      
Disclosure of detailed information about financial instruments [line items]      
Cash and cash equivalents 0 0  
Borrowings 4,300 4,400  
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 178 146  
Level 1      
Disclosure of detailed information about financial instruments [line items]      
Bonds, carrying value 6,700 7,300  
Bonds, fair value 7,000 8,700  
Level 1 | Fair value      
Disclosure of detailed information about financial instruments [line items]      
Cash and cash equivalents $ 3,916 $ 4,138