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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (1,028) $ (2,273) $ (1,730)
– Deficits (2,098) (3,055)  
– Surpluses 1,070 782  
Net deficits on pension plans (194) (1,305)  
Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (194) (1,305)  
– Deficits (1,264)    
– Surpluses 1,070    
Net deficits on pension plans (194)    
Other benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (834) (968)  
– Deficits (834)    
– Surpluses 0    
Net deficits on pension plans 0    
Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (834) (968)  
Unfunded | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation 0    
Unfunded | Other benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (834)    
Plan assets      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 14,700 14,905 13,923
Plan assets | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 14,700 14,905  
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 (15,728) (17,178) $ (15,653)
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 (14,894) (16,210)  
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 (834) (968)  
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 (14,462) (15,731)  
Present value of defined benefit obligation | Funded | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (14,462)    
Present value of defined benefit obligation | Funded | Other benefits      
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 | Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (1,266) $ (1,447)  
Present value of defined benefit obligation | Unfunded | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (432)    
Present value of defined benefit obligation | Unfunded | Other benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (834)