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Employee Future Benefits - Components of Net Benefit Cost for Pension Plans and Retiree Welfare Plans (Detail) - Net Benefit Cost Recognized in the Consolidated Statements of Income - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Pension plans    
Disclosure of defined benefit plans [line items]    
Defined benefit current service cost $ 44 $ 41
Defined benefit administrative expenses 8 10
Past service cost – plan amendments and curtailments 0 0
Service cost 52 51
Interest on net defined benefit (asset) liability 4 5
Defined benefit cost 56 56
Defined contribution cost 97 93
Net benefit cost 153 149
Retiree welfare plans    
Disclosure of defined benefit plans [line items]    
Defined benefit current service cost 0 0
Defined benefit administrative expenses 2 1
Past service cost – plan amendments and curtailments 0 0
Service cost 2 1
Interest on net defined benefit (asset) liability (5) (3)
Defined benefit cost (3) (2)
Defined contribution cost 0 0
Net benefit cost $ (3) $ (2)