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Employee Future Benefits - Summary of Amounts Recognized in Consolidated Statements of Financial Position (Detail) - 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 obligation $ 3,789 $ 3,794
Fair value of plan assets 3,706 3,722
Deficit 83 72
Effect of asset limit 41 48
Deficit and net defined benefit liability 124 120
Pension defined benefit plans [member] | Funded [Member]    
Disclosure of defined benefit plans [Line Items]    
Deficit and net defined benefit liability (422) (441)
Pension defined benefit plans [member] | Unfunded [Member]    
Disclosure of defined benefit plans [Line Items]    
Deficit and net defined benefit liability 546 561
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit obligation 450 466
Fair value of plan assets 526 523
Deficit (76) (57)
Effect of asset limit   0
Deficit and net defined benefit liability (76) (57)
Retiree welfare plans [Member] | Funded [Member]    
Disclosure of defined benefit plans [Line Items]    
Deficit and net defined benefit liability (190) (168)
Retiree welfare plans [Member] | Unfunded [Member]    
Disclosure of defined benefit plans [Line Items]    
Deficit and net defined benefit liability $ 114 $ 111