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Components of Net Periodic Benefit Cost (Detail) - CAD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Pension benefits        
Defined Benefit Plan Disclosure [Line Items]        
Current service cost $ 60 $ 54 $ 120 $ 109
Interest cost 75 79 151 158
Expected return on plan assets (100) (101) (201) (202)
Amortization of prior service cost 1 2 2 5
Amortization of actuarial loss (gain) 43 45 87 89
Net periodic benefit cost 79 79 159 159
Other postretirement benefits        
Defined Benefit Plan Disclosure [Line Items]        
Current service cost 4 4 8 8
Interest cost 6 6 11 12
Amortization of actuarial loss (gain) 1 2 3 4
Net periodic benefit cost $ 11 $ 12 $ 22 $ 24