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Employee Future Benefits - Summary of Components of Net Benefit Cost for Pension Plans and Retiree Welfare Plans (Detail) - Net Benefit Cost Recognized in the Consolidated Statements of Income [Member] - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost $ 41 $ 43
Defined benefit administrative expenses 10 11
Past service cost-plan amendments and curtailments 0 (6)
Service cost 51 48
Interest on net defined benefit (asset) liability 5 2
Defined benefit cost 56 50
Defined contribution cost 93 85
Net benefit cost 149 135
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost 0  
Defined benefit administrative expenses 1 2
Past service cost-plan amendments and curtailments 0  
Service cost 1 2
Interest on net defined benefit (asset) liability (3)  
Defined benefit cost (2) 2
Defined contribution cost 0  
Net benefit cost $ (2) $ 2