Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedge - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains and (losses) on derivatives	$ (47,881)		$ (48,763)
Natural gas swaps | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(25,919)		(22,848)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	11
Losses	(4,228)	$ (5,527)
Total gains (losses) on derivatives	(4,217)	$ (5,527)
Interest rate options | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	$ (21,962)		$ (25,915)