v3.25.3
Employee retirement benefits - Components of Net Periodic Benefit Cost (Detail) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Pension benefits:        
Service cost $ 46 $ 47 $ 139 $ 139
Interest cost 93 91 277 274
Expected return on plan assets (99) (113) (296) (340)
Amortization of prior service cost (credit) 7 7 21 21
Amortization of actuarial loss (gain) 2 11 8 35
Net pension enhancement 19 0 19 0
Net benefit cost 68 43 168 129
Other postretirement benefits:        
Service cost 2 3 4 10
Interest cost 4 7 15 19
Amortization of prior service cost (credit) (1) 0 (3) 0
Amortization of actuarial loss (gain) (1) (2) (6) (6)
Net pension enhancement 1 0 1 0
Net benefit cost $ 5 $ 8 $ 11 $ 23