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Financial instruments disclosures Additional information (Detail) - USD ($)
 6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items]      
Increase in coal carrying amount $ 195,000,000 $ 181,000,000  
Decrease in coal carrying amount 39,000,000 95,000,000  
Level 1      
Disclosure of detailed information about financial instruments [line items]      
Borrowings 9,000,000,000 8,300,000,000  
Level 3      
Disclosure of detailed information about financial instruments [line items]      
Borrowings $ 4,700,000,000 4,600,000,000  
Change in London Metal Exchange (as a percent) 10.00%    
Period of London metal exchange curve (in years) P10Y    
Decrease in carrying value of forward contract due to long-term metal pricing assumptions $ 17,000,000 22,000,000  
Increase in carrying value of forward contract due to long-term metal pricing assumptions 2,000,000   $ 14,000,000
Bonds      
Disclosure of detailed information about financial instruments [line items]      
Borrowings 7,700,000,000 7,500,000,000  
Project Finance Drawn Down      
Disclosure of detailed information about financial instruments [line items]      
Borrowings 4,200,000,000 $ 4,200,000,000  
Minimum      
Disclosure of detailed information about financial instruments [line items]      
Aluminium Market Price 2,473    
Maximum      
Disclosure of detailed information about financial instruments [line items]      
Aluminium Market Price $ 2,556