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Provisions, including post-retirement benefits - Summary of Provisions, including post-retirement benefits (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of other provisions [Line Items]      
Remaining lives of operations and infrastructure present closure obligation 20 years 16 years  
Remaining lives of operations and infrastructure one to over 50 years    
Close-down and restoration costs and environmental clean-up obligations $ 9,983 $ 8,722  
Risk free interest rate on provisions 2.00%    
Provision $ 14,642 13,794 $ 13,066
Non-current provisions $ 13,367 12,479  
Bottom of range [Member]      
Disclosure of other provisions [Line Items]      
Risk free interest rate on provisions 0.50%    
Top of range [Member]      
Disclosure of other provisions [Line Items]      
Provision $ 1,102    
Other employee entitlements [member]      
Disclosure of other provisions [Line Items]      
Provision for long service leave 292 317  
Provision for redundancy and severance payments 24 59  
Provision 389 450  
Non-current provisions 112    
Close down and restoration/ environmental [member]      
Disclosure of other provisions [Line Items]      
Provision 9,983 8,722  
Non-current provisions 9,349    
Provisions relating to insurance recoveries and other financial assets held 75 110  
Close down and restoration/ environmental [member] | Pre 2021 [Member]      
Disclosure of other provisions [Line Items]      
Non-current provisions 336 366  
Close down and restoration/ environmental [member] | More than 5 years [member]      
Disclosure of other provisions [Line Items]      
Non-current provisions $ 839 $ 727