v3.25.4
Employee Future Benefits - Potential Impact on Obligations Arising From Changes in Key Assumptions (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
U.S.    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Decrease in mortality rate 10.00%  
U.S. | Males    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Increase in life expectancy due to decrease in mortality rate 9 months 18 days  
U.S. | Females    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Increase in life expectancy due to decrease in mortality rate 9 months 18 days  
Discount rate | Pension plans    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Impact of increase in actuarial assumption $ (238) $ (260)
Impact of decrease in actuarial assumption 279 305
Discount rate | Retiree welfare plans    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Impact of increase in actuarial assumption (30) (34)
Impact of decrease in actuarial assumption 35 40
Health care cost trend rate | Retiree welfare plans    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Impact of increase in actuarial assumption 8 9
Impact of decrease in actuarial assumption (7) (8)
Mortality rates | Pension plans    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Impact of decrease in actuarial assumption 88 89
Mortality rates | Retiree welfare plans    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Impact of decrease in actuarial assumption $ 6 $ 9