Derivative Instruments (Details 4) - Not designated as hedges - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains and (losses) on derivatives	$ (52,265)		$ (52,265)		$ (68,146)
Natural gas swaps | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(15,942)		(15,942)		(18,914)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	181	$ 956	181	$ 1,236
Losses	(3,248)		(8,775)
Total gains (losses) on derivatives	(3,067)	$ 956	(8,594)	$ 1,236
Interest rate options | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	$ (36,323)		$ (36,323)		$ (49,232)