Disclosures about Derivative Instruments and Hedging Activities (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total losses on derivatives	$ (25,276)	$ (70,403)
Designated as hedges | Natural gas swaps | Receivables
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses		(5,244)
Gains	4,336
Designated as hedges | Natural gas swaps | Regulatory assets
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses	(11)	(11)
Gains	990	990
Designated as hedges | Natural gas swaps | Purchased power
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses	(3,095)	(5,502)
Gains	149	149
Not designated as hedges | Interest rate options | Regulatory assets
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses	$ (27,645)	$ (60,785)