Derivative Accounting (Details 2) (Commodity Contracts, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Designated as Hedging Instruments
Gains and losses from derivative instruments
Loss Recognized in OCI on Derivative Instruments (Effective Portion)	$ (37,663,000)	$ (94,660,000)	$ (155,287,000)
Loss Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion Realized)	(99,007,000)	(117,189,000)	(122,740,000)
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)	117,000	(211,000)	3,680,000
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	44,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(2,644,000)	(52,140,000)	(106,254,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	103,000	(27,000)	1,436,000
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ (2,747,000)	$ (52,113,000)	$ (107,690,000)