Derivative Instruments (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended				3 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2013 Not designated as hedges	Sep. 30, 2013 Not designated as hedges	Dec. 31, 2012 Not designated as hedges	Sep. 30, 2013 Not designated as hedges Natural gas swaps Regulatory asset	Dec. 31, 2012 Not designated as hedges Natural gas swaps Regulatory asset	Sep. 30, 2013 Not designated as hedges Natural gas swaps Fuel	Sep. 30, 2013 Not designated as hedges Natural gas swaps Fuel	Sep. 30, 2013 Not designated as hedges Interest rate options Regulatory asset	Dec. 31, 2012 Not designated as hedges Interest rate options Regulatory asset	Sep. 30, 2012 Designated as hedges	Sep. 30, 2012 Designated as hedges	Sep. 30, 2012 Designated as hedges Natural gas swaps Fuel	Sep. 30, 2012 Designated as hedges Natural gas swaps Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains						$ 122	$ 688					$ 173	$ 197
Losses						(3,089)	(4,002)					(3,934)	(9,204)
Total gains (losses) on derivatives	(2,967)	(3,314)								(3,761)	(9,007)
Unrealized losses on derivatives				(197)	(1,085)			(41,544)	(74,217)
Unrealized gains and (losses) on derivatives	$ (41,741)	$ (41,741)	$ (75,302)