Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total unrealized gains (losses)	$ (6,903)		$ (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	(6,903)		$ (6,328)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	1,392	$ 840
Losses	(748)	(744)
Total	$ 644	$ 96