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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
CAD in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost CAD 48 CAD 52
Defined benefit administrative expenses 5 7
Past service cost - plan amendments   (57)
Service cost 53 2
Interest on net defined benefit (asset) liability 18 27
Defined benefit cost 71 29
Defined contribution cost 75 69
Net benefit cost 146 98
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Defined benefit current service cost 1 1
Defined benefit administrative expenses 2 2
Service cost 3 3
Interest on net defined benefit (asset) liability 3 3
Defined benefit cost 6 6
Net benefit cost CAD 6 CAD 6