Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total unrealized gains (losses)	$ (11,382)		$ (11,382)		$ (6,328)
Natural gas swaps | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(11,382)		(11,382)		$ (6,328)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	359	$ 1,897	1,751	$ 2,736
Losses	(111)	(73)	(859)	(817)
Total	$ 248	$ 1,824	$ 892	$ 1,919