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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (2,273) $ (1,730) $ (1,551)
Deficits (3,055) (2,714)  
Surpluses 782 984  
Net deficits on pension plans (1,305) (831)  
Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (968) (899)  
Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (1,305) (831)  
Deficits (2,087)    
Surpluses 782    
Net deficits on pension plans (1,305)    
Pension benefits | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation 0    
Other Benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (968) (899)  
Deficits (968)    
Surpluses 0    
Net deficits on pension plans 0    
Other Benefits | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (968)    
Plan assets      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 14,905 13,923 13,203
Plan assets | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 14,905 13,923  
Plan assets | Other Benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 0 0  
Present value of defined benefit obligation      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (17,178) (15,653) $ (14,754)
Present value of defined benefit obligation | Funded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (15,731) (14,311)  
Present value of defined benefit obligation | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (1,447) (1,342)  
Present value of defined benefit obligation | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (16,210) (14,754)  
Present value of defined benefit obligation | Pension benefits | Funded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (15,731)    
Present value of defined benefit obligation | Pension benefits | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (479)    
Present value of defined benefit obligation | Other Benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (968) $ (899)  
Present value of defined benefit obligation | Other Benefits | Funded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 0    
Present value of defined benefit obligation | Other Benefits | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (968)