v3.26.1
Employee retirement benefits - Components of Net Periodic Benefit Cost (Detail) - CAD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Pension benefits:        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 44 $ 46 $ 87 $ 93
Interest cost 95 91 190 184
Expected return on plan assets (100) (98) (201) (197)
Amortization of prior service cost (credit) 6 8 13 14
Amortization of actuarial loss (gain) 0 3 0 6
Net benefit cost 45 50 89 100
Other postretirement benefits:        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1 1 2 2
Interest cost 5 6 11 11
Amortization of prior service cost (credit) (1) (1) (2) (2)
Amortization of actuarial loss (gain) (2) (3) (5) (5)
Curtailment loss (gain) (1) 0 (1) 0
Net benefit cost $ 2 $ 3 $ 5 $ 6