Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Designated as Hedging Instruments
Gains and losses from derivative instruments
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion)	$ 96,000	$ (41,903,000)
Loss Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion Realized)	(8,353,000)	(14,500,000)
Gain Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)		85,000
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income	40,000,000
Gains and losses from derivative instruments
Amount reclassified from AOCI to earnings related to discontinued cash flow hedges	0	0
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	17,233,000	(25,378,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(117,000)	(326,000)
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ 17,350,000	$ (25,052,000)