Derivative instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedges - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total unrealized gains (losses)	$ (807)		$ (807)		$ 9,302
Natural gas swaps | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(2,788)		(2,788)		(62)
Natural gas swaps | Regulatory liability
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains on derivatives	1,981		1,981		15,152
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	778	$ 2,039	3,514	$ 2,057
Losses	(678)	(5,923)	(1,495)	(18,262)
Total gains (losses) on derivatives	$ 100	$ (3,884)	$ 2,019	$ (16,205)
Interest rate options | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives					$ (5,788)