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Components of Net Periodic Benefit Cost (Detail) - CAD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Pension benefits    
Service cost $ 70 $ 81
Interest cost 73 68
Expected return on plan assets (103) (107)
Amortization of prior service cost 4 4
Amortization of actuarial loss (gain) 22 36
Net benefit cost 66 82
Other postretirement benefits    
Service cost 6 7
Interest cost 6 6
Amortization of actuarial loss (gain) 2 4
Net benefit cost $ 14 $ 17