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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (1,730) $ (1,551) $ (2,499)
Effect of asset ceiling   0  
Deficits (2,714) (2,486)  
Surpluses 984 935  
Net deficits on pension plans (831) (680)  
Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (899) (871)  
Pension benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (831) (680)  
Deficits (1,815)    
Surpluses 984    
Net deficits on pension plans (831)    
Other Benefits [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (899) (871)  
Deficits (899)    
Other Benefits [member] | Unfunded [member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (899)    
Plan assets [Member]      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 13,923 13,203 15,257
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 13,923 13,203  
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 (15,653) (14,754) $ (17,645)
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 (14,311) (13,482)  
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 (1,342) (1,272)  
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 (14,754) (13,883)  
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 (14,311)    
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 (443)    
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 (899) $ (871)  
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 $ (899)