Derivative Instruments - Notional Amounts of Outstanding Derivative Instruments (Details) ¥ in Thousands, $ in Thousands, $ in Thousands	Sep. 30, 2018 USD ($) MW	Sep. 30, 2018 CAD ($) MW	Sep. 30, 2018 JPY (¥) MW	Dec. 31, 2017 USD ($) MW	Dec. 31, 2017 CAD ($) MW	Dec. 31, 2017 JPY (¥) MW	Dec. 31, 2010 MW
Interest rate swaps [Member] | Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount	$ 419,633	$ 725,531	¥ 55,806,344	$ 253,271	$ 736,136	¥ 0
Interest rate swaps [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount | $	$ 61,032			$ 85,474
Energy derivative [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Nonmonetary notional amount | MW	313,948	313,948	313,948	697,471	697,471	697,471	504,220
Foreign currency forward contracts [Member] | Not Designated as Hedging Instrument [Member]
Notional Amount of Outstanding Derivative Contracts [Line Items]
Notional amount		$ 105,750	¥ 11,786,050		$ 127,500	¥ 0