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Employee Benefit Plans (Benefit Cost Components) (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Service Cost $ 49 $ 35
Defined Benefit Plan, Interest Cost 99 79
Defined Benefit Plan, Expected Return on Plan Assets (129) (97)
Defined Benefit Plan, Amortization of Gains (Losses) 38 42
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 0 (1)
Current year amortization of regulatory asset liabilities 17 9
Settlement, curtailment (1) 0
Net Periodic Benefit Cost, Total 73 67
Non-pension Benefit Plans [Member    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Service Cost 5 4
Defined Benefit Plan, Interest Cost 14 9
Defined Benefit Plan, Expected Return on Plan Assets (3) (1)
Defined Benefit Plan, Amortization of Gains (Losses) 2 2
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (8) (8)
Current year amortization of regulatory asset liabilities (1) 0
Settlement, curtailment 0 0
Net Periodic Benefit Cost, Total $ 9 $ 6