Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Designated as Hedging Instruments
Gains and losses from derivative instruments
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion)	$ 4,509,000	$ (16,323,000)	$ (37,394,000)	$ (15,335,000)
Loss Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion Realized)	(23,015,000)	(25,287,000)	(37,515,000)	(40,133,000)
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)	32,000	(176,000)	117,000	(164,000)
Amount reclassified from AOCI to earnings related to discontinued cash flow hedges	1,800,000		1,800,000
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income			77,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	26,129,000	(3,395,000)	751,000	(10,915,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	87,000	(503,000)	(239,000)	1,004,000
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ 26,042,000	$ (2,892,000)	$ 990,000	$ (11,919,000)