v3.26.1
Employee retirement benefits - Components of Net Periodic Benefit Cost (Detail) - CAD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Pension benefits:    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 43 $ 47
Interest cost 95 93
Expected return on plan assets (101) (99)
Amortization of prior service cost (credit) 7 6
Amortization of actuarial loss (gain) 0 3
Net benefit cost 44 50
Other postretirement benefits:    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 1 1
Interest cost 6 5
Amortization of prior service cost (credit) (1) (1)
Amortization of actuarial loss (gain) (3) (2)
Net benefit cost $ 3 $ 3