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Financial instruments disclosures - Additional information (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]      
Borrowings $ 12,604   $ 12,806
Expected unconditional guaranteed royalty payments to be received $ 111 $ 124  
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 $ 444   151
Hedged item, liabilities 281   298
Interest rate swap contract      
Disclosure of detailed information about financial instruments [line items]      
Notional amount 1,500    
Level 1      
Disclosure of detailed information about financial instruments [line items]      
Bonds, carrying value 7,500   7,700
Bonds, fair value 9,200   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 $ 177   214
Decrease in carrying value of forward contract due to decrease in spot price 44   57
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 326   120
Level 3 | Fair value      
Disclosure of detailed information about financial instruments [line items]      
Borrowings $ 4,700   $ 4,700