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Changes in the Standardised Measure (Parenthetical) (Detail) - USD ($)
$ in Millions
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2017
Discounted future net cash flows relating to proved oil and gas reserves [line items]        
Standardised measure $ 5,058 $ 9,152 $ 10,240 $ 8,242
United States [member]        
Discounted future net cash flows relating to proved oil and gas reserves [line items]        
Standardised measure 2,346 3,590 5,257  
Discontinued operations - Onshore US Cash Generating Units [member] | United States [member]        
Discounted future net cash flows relating to proved oil and gas reserves [line items]        
Standardised measure $ 0 $ 0 $ 1,932