Derivative Instruments - Notional Amounts of Outstanding Derivative Instruments (Details) CAD in Thousands, $ in Thousands	Jun. 30, 2017 USD ($) MW	Jun. 30, 2017 CAD MW	Dec. 31, 2016 USD ($) MW	Dec. 31, 2016 CAD MW	Dec. 31, 2010 MW
Interest rate swaps [Member] | Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount	$ 433,849	CAD 195,975	$ 365,443	CAD 196,425
Interest rate swaps [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount | $	$ 250,488		$ 257,389
Energy derivative [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Nonmonetary notional amount | MW	911,048	911,048	1,201,691	1,201,691	504,220
Foreign currency forward contracts [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount | CAD		CAD 142,750		CAD 95,800