Disclosures about Derivative Instruments and Hedging Activities (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Designated as hedges | Natural gas swaps
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total net gain (loss)	$ (2,517)	$ (7,870)
Total net unrealized gain (loss)	(13)	(13)
Designated as hedges | Natural gas swaps | Receivables
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total net unrealized gain (loss)	(634)	(634)
Designated as hedges | Natural gas swaps | Regulatory assets
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total net unrealized gain (loss)	621	621
Designated as hedges | Natural gas swaps | Purchased power
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses	(3,934)	(9,204)
Gains	173	197
Designated as hedges | Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Losses	(83)	(315)
Gains	1,327	1,452
Not designated as hedges | Interest rate options | Regulatory assets
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total net unrealized gain (loss)	$ (70,079)	$ (70,079)