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Employee Future Benefits - Summary of Disaggregation of Defined Benefit Obligation (Detail) - CAD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation $ 4,560 $ 4,901
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 584 638
U.S. plans [Member] | Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 2,908 3,079
U.S. plans [Member] | Pension defined benefit plans [member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 537 551
U.S. plans [Member] | Pension defined benefit plans [member] | Inactive and retired members [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 2,371 2,528
U.S. plans [Member] | Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 433 472
U.S. plans [Member] | Retiree welfare plans [Member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 17 27
U.S. plans [Member] | Retiree welfare plans [Member] | Inactive and retired members [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 416 445
Canadian plans [Member] | Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 1,652 1,822
Canadian plans [Member] | Pension defined benefit plans [member] | Active [member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 184 211
Canadian plans [Member] | Pension defined benefit plans [member] | Inactive and retired members [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 1,468 1,611
Canadian plans [Member] | Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation 151 166
Canadian plans [Member] | Retiree welfare plans [Member] | Inactive and retired members [Member]    
Disclosure of defined benefit plans [Line Items]    
Disaggregation of defined benefit obligation $ 151 $ 166