Financial and Other Derivative Instruments (Cumulative Gross Volume of Derivative Contracts Outstanding) (Details)	9 Months Ended
	Sep. 30, 2017 CAD MWh MMBTU
Natural Gas (MMBtu)
Derivative [Line Items]
Commodity | MMBTU	1,751,191,598
Electricity (MWh)
Derivative [Line Items]
Commodity | MWh	28,924,416
Foreign Currency Exchange (Canadian dollars)
Derivative [Line Items]
Foreign Currency Exchange (Canadian dollars) | CAD	CAD 89,049,511