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Fair Values (Hybrid Notes) (Details) - Designated as Hedging Instrument [Member] - Cash Flow Hedging [Member] - Foreign Currency Gain (Loss) [Member]
$ in Billions
12 Months Ended
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Dec. 31, 2017
USD ($)
Hybrid Instruments [Line Items]      
Hybrid Instruments at Fair Value, Net     $ 1.2
Other Comprehensive Income (Loss) [Member]      
Hybrid Instruments [Line Items]      
Unrealized Gain (Loss) on Foreign Currency Derivatives, Net, before Tax $ 97,000,000 $ 49,000,000  
Gain (Loss) on Foreign Currency Cash Flow Hedge Ineffectiveness $ 0 $ 0