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Retirement Benefit Plans - Additional Information (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]    
Surplus(deficit)of plan $ 134 $ 44
Expected contribution to defined benefit pension plans for the next annual reporting period 22  
Defined contribution expense $ 47 44
Non-Pension Post-Retirement Benefit [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Weighted average duration of the defined benefit obligation 16 years  
Non-Pension Post-Retirement Benefit [member] | British Columbia [member] | Medical Services Plan Premiums [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Effect from change in financial assumptions $ 19 $ 104
Defined Benefit Pension Plans [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Weighted average duration of the defined benefit obligation 14 years