Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Designated as Hedging Instruments
Gains and losses from derivative instruments
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion)	$ (41,903,000)	$ 988,000
Loss Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion Realized)	(14,500,000)	(14,847,000)
Gain Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)	85,000	12,000
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income	97,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(25,378,000)	(7,519,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(326,000)	1,507,000
Not Designated as Hedging Instruments | Fuel and purchased power expense
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ (25,052,000)	$ (9,026,000)