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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (2,499) $ (2,542) $ (2,530)
Deficits (3,370) (3,167)  
Surpluses 871 625  
Net deficits on pension plans (1,501) (1,616)  
Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (998) (926)  
Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (1,501) (1,616)  
Deficits (2,372)    
Surpluses 871    
Net deficits on pension plans (1,501)    
Other Benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (998) (926)  
Deficits (998)    
Other Benefits [member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (998)    
Plan assets [Member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 15,257 13,749 13,642
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 15,257 13,749  
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 (17,645) (16,228) (16,123)
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 (16,199) (14,504)  
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 (1,446) (1,724)  
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 (16,647) (15,302)  
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 (16,199)    
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 (448)    
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 (998) (926)  
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 (998)    
Effect of asset ceiling [Member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (111) (63) $ (49)
Effect of asset ceiling [Member] | Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (111) $ (63)