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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (723) $ (470) $ (1,028)
Effect of asset ceiling (66) (1)  
– Deficits (1,189) (1,294)  
– Surpluses 466 824  
Net (deficit)/surplus on pension plans (95) 152  
Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (95) 152  
Effect of asset ceiling (66)    
– Deficits (561)    
– Surpluses 466    
Net (deficit)/surplus on pension plans (95)    
Other benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (628) (622)  
Effect of asset ceiling 0    
– Deficits (628)    
– Surpluses 0    
Net (deficit)/surplus on pension plans 0    
Unfunded      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Unfunded post-retirement healthcare obligation (628) (622)  
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 (628)    
Plan assets      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 11,138 10,708 14,700
Plan assets | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 11,138 10,708  
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 (11,795) (11,177) $ (15,728)
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 (11,167) (10,555)  
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 (628) (622)  
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 (10,799) (10,226)  
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 (10,799)    
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 (996) $ (951)  
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 (368)    
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 $ (628)