Financial and Other Derivative Instruments (Cumulative Gross Volume of Derivative Contracts Outstanding) (Details)	3 Months Ended
	Mar. 31, 2018 CAD ($) MMBTU MWh
Natural gas (MMBtu)
Derivative [Line Items]
Commodity | MMBTU	1,858,053,782
Electricity (MWh)
Derivative [Line Items]
Commodity | MWh	38,397,695
Foreign currency exchange (Canadian dollars)
Derivative [Line Items]
Foreign currency exchange (Canadian dollars) | $	$ 103,281,830