Financial and Other Derivative Instruments (Cumulative Gross Volume of Derivative Contracts Outstanding) (Details)	12 Months Ended
	Dec. 31, 2017 CAD MWh MMBTU
Natural Gas (MMBtu)
Derivative [Line Items]
Commodity | MMBTU	1,822,796,301
Electricity (MWh)
Derivative [Line Items]
Commodity | MWh	29,715,268
Foreign Currency Exchange (Canadian dollars)
Derivative [Line Items]
Foreign Currency Exchange (Canadian dollars) | CAD	CAD 108,023,948