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Employee Future Benefits - Summary of Components of Net Benefit Cost for Pension Plans and Retiree Welfare Plans (Detail) - Net Benefit Cost Recognized in the Consolidated Statements of Income [Member] - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost $ 42 $ 48
Defined benefit administrative expenses 6 5
Past service cost - plan amendments 8  
Past service cost - curtailments 10  
Service cost 66 53
Interest on net defined benefit (asset) liability 12 18
Defined benefit cost 78 71
Defined contribution cost 78 75
Net benefit cost 156 146
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost   1
Defined benefit administrative expenses 2 2
Past service cost - curtailments 12  
Service cost 14 3
Interest on net defined benefit (asset) liability 3 3
Defined benefit cost 17 6
Net benefit cost $ 17 $ 6