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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet $ (658) $ (723) $ (470)
Effect of asset ceiling (50) (66)  
– Deficits (1,063) (1,189)  
– Surpluses 405 466  
Net deficits on pension plans (82) (95)  
Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (82) (95)  
Effect of asset ceiling (50)    
– Deficits (487)    
– Surpluses 405    
Net deficits on pension plans (82)    
Other benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet (576) (628)  
Effect of asset ceiling 0    
– Deficits (576)    
– 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 (576) (628)  
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 (576)    
Plan assets      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 10,155 11,138 10,708
Plan assets | Pension benefits      
Disclosure of information about amounts recognized in balance sheet [Line Items]      
Net deficit to be shown in the balance sheet 10,155 11,138  
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 (10,763) (11,795) $ (11,177)
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 (10,187) (11,167)  
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 (576) (628)  
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 (9,840) (10,799)  
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 (9,840)    
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 (923) $ (996)  
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 (347)    
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 $ (576)