Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Designated as Hedging Instruments
Gains and losses from derivative instruments
Loss Recognized in OCI on Derivative Instruments (Effective Portion)	$ (119,000)	$ (25,457,000)	$ (37,513,000)	$ (40,792,000)
Loss Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion Realized)	(49,478,000)	(59,144,000)	(86,993,000)	(99,278,000)
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)		17,000	117,000	(147,000)
Amount reclassified from AOCI to earnings related to discontinued cash flow hedges			(1,800,000)
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income			(51,000,000)
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain Recognized in Income from Derivative Instruments	13,128,000	(13,138,000)	13,879,000	(24,053,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain Recognized in Income from Derivative Instruments	258,000	81,000	19,000	1,085,000
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain Recognized in Income from Derivative Instruments	$ 12,870,000	$ (13,219,000)	$ 13,860,000	$ (25,138,000)