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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (1,551) $ (2,499) $ (2,542)
Effect of asset ceiling   (111)  
Deficits (2,486) (3,370)  
Surpluses 935 871  
Net deficits on pension plans (680) (1,501)  
Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (871) (998)  
Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (680) (1,501)  
Deficits (1,615)    
Surpluses 935    
Net deficits on pension plans (680)    
Other Benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (871) (998)  
Deficits (871)    
Other Benefits [member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (871)    
Plan assets [Member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 13,203 15,257 13,749
Plan assets [Member] | Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 13,203 15,257  
Present value of defined benefit obligation [Member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (14,754) (17,645) $ (16,228)
Present value of defined benefit obligation [Member] | Funded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (13,482) (16,199)  
Present value of defined benefit obligation [Member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (1,272) (1,446)  
Present value of defined benefit obligation [Member] | Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (13,883) (16,647)  
Present value of defined benefit obligation [Member] | Pension benefits [member] | Funded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (13,482)    
Present value of defined benefit obligation [Member] | Pension benefits [member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (401)    
Present value of defined benefit obligation [Member] | Other Benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (871) $ (998)  
Present value of defined benefit obligation [Member] | Other Benefits [member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (871)