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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of other provisions [Line Items]      
Remaining lives of operations and infrastructure one to over 60 years    
Remaining lives of operations and infrastructure present closure obligation 18 years P17Y  
Provision for carrying value close-down, restoration and environmental   $ 9,975  
Risk free interest rate on provisions 2.00%    
Provisions $ 15,103 13,608 $ 14,642
Non-current provisions $ 13,704 12,552  
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]      
Provisions $ 979    
Other employee entitlements [member]      
Disclosure of other provisions [Line Items]      
Provision for long service leave 248 242  
Provision for redundancy and severance payments 30 46  
Provisions 354 360  
Non-current provisions 94    
Close down and restoration/ environmental [member]      
Disclosure of other provisions [Line Items]      
Provisions 11,090 9,975  
Non-current provisions 10,549    
Provisions relating to insurance recoveries and other financial assets held 113 110  
Close down and restoration/ environmental [member] | Between 1 and 5 years [member]      
Disclosure of other provisions [Line Items]      
Non-current provisions 382 535  
Close down and restoration/ environmental [member] | More than 5 years [member]      
Disclosure of other provisions [Line Items]      
Non-current provisions $ 883 $ 683