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Financial Risk Management - Additional Information (Detail) - USD ($)
6 Months Ended
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Transfer between fair value hierarchy categories $ 0    
Commodity price risk [member] | Level 3 [member] | Shares and other investments available for sale and other derivative contracts [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Reasonable possible change in risk, percentage 10.00%   10.00%
Effect on equity and profit after taxation, due to reasonable possible change in risk $ 45,000,000   $ 50,000,000
Fixed interest rate not swapped to floating rate [member]      
Disclosure of nature and extent of risks arising from 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 nature and extent of risks arising from financial instruments [line items]      
Borrowings $ 3,366,000,000 $ 3,434,000,000