Financial and Other Derivative Instruments (Cumulative Gross Volume of Derivative Contracts Outstanding) (Details)	6 Months Ended
	Jun. 30, 2018 CAD ($) MWh MMBTU
Natural gas (MMBtu)
Derivative [Line Items]
Commodity | MMBTU	1,908,431,324
Electricity (MWh)
Derivative [Line Items]
Commodity | MWh	39,463,885
Foreign currency exchange (Canadian dollars)
Derivative [Line Items]
Foreign currency exchange (Canadian dollars) | $	$ 101,077,223