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Financial and Other Derivative Instruments (Cumulative Gross Volume of Derivative Contracts Outstanding) (Details)
3 Months Ended
Mar. 31, 2017
CAD
MWh
MMBTU
gal
Natural Gas (MMBtu)  
Derivative [Line Items]  
Commodity | MMBTU 1,781,816,780
Electricity (MWh)  
Derivative [Line Items]  
Commodity | MWh 35,032,431
Oil (Gallons)  
Derivative [Line Items]  
Oil (Gallons) | gal 12,936,000
Foreign currency exchange contracts (Canadian dollars)  
Derivative [Line Items]  
Foreign Currency Exchange (Canadian dollars) | CAD CAD 73,633,072