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Employee Future Benefits - Summary of Cash Payments (Detail) - CAD
CAD in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans CAD 160 CAD 175
Pension plans [Member] | Defined benefit plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 85 106
Pension plans [Member] | Defined contribution plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 75 CAD 69
Retiree welfare plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 12  
Retiree welfare plans [Member] | Defined benefit plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans CAD 12