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Financial Risk Management - Additional Information (Detail) - USD ($)
6 Months Ended
Dec. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]      
Transfer between fair value hierarchy categories $ 0    
Commodity price risk [member] | Shares and other investments available for sale and other derivative contracts [Member] | Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Reasonable possible change in risk, percentage     10.00%
Increase/(decrease) profit after taxation, due to reasonable possible change in risk 36,000,000   $ 45,000,000
Fixed interest rate not swapped to floating rate [member]      
Disclosure of detailed information about financial instruments [line items]      
Borrowings 3,019,000,000 $ 3,019,000,000  
Fixed interest rate not swapped to floating rate [member] | At fair value [member]      
Disclosure of detailed information about financial instruments [line items]      
Borrowings $ 3,847,000,000 $ 3,757,000,000