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Employee Future Benefits - Summary of Cash Payments (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans $ 152 $ 144
Retiree welfare plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 12 11
Defined benefit plans [Member] | Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 59 59
Defined benefit plans [Member] | Retiree welfare plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 12 11
Defined contribution plans [Member] | Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 93 85
Defined contribution plans [Member] | Retiree welfare plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans $ 0 $ 0