v3.26.1
PENSION PLANS AND POSTRETIREMENT BENEFITS - Assumptions and sensitivity analysis (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Current periodic costs      
Percentage increase of discount rate (as a percent) 0.10%    
Pension and postretirement benefits      
Current periodic costs      
Decrease of retirement benefit obligation due to increase of actuarial assumption $ 9.5    
Actuarial assumption discount rate      
Current periodic costs      
Assumed average retirement age 62 years 62 years 62 years
Assumed health care cost trend rate (as a percent) 5.40%    
Assumed health care cost trend rate expected to decrease gradually over the years (as a percent) 4.10%    
Actuarial assumption discount rate | Pension and postretirement benefits      
Benefit obligations      
Discount rate, current year (as a percent) 4.90% 4.70% 4.60%
Rate of compensation increase, current year (as a percent) 3.10% 3.10% 3.00%
Current periodic costs      
Discount rate, preceding year (as a percent) 4.70% 4.60% 5.10%
Rate of compensation increase, preceding year (as a percent) 3.10% 3.00% 3.00%