Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedge - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total unrealized gains (losses)	$ (14,256)		$ (14,256)		$ (48,763)
Natural gas swaps | Regulatory liability
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains on derivatives	381		381
Natural gas swaps | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(3,204)		(3,204)		(22,848)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	2,039	$ 24	2,057	$ 205
Losses	(5,923)	(5,970)	(18,262)	(14,744)
Total gains (losses) on derivatives	(3,884)	$ (5,946)	(16,205)	$ (14,539)
Interest rate options | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	$ (11,433)		$ (11,433)		$ (25,915)