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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, 2021
Dec. 31, 2020
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost $ 44 $ 41
Defined benefit administrative expenses 9 7
Service cost 53 48
Interest on net defined benefit (asset) liability 6 10
Defined benefit cost 59 58
Defined contribution cost 90 84
Net benefit cost 149 142
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit administrative expenses 2 2
Service cost 2 2
Interest on net defined benefit (asset) liability 1 1
Defined benefit cost 3 3
Net benefit cost $ 3 $ 3