Derivative Accounting (Details 2) (Commodity Contracts, USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Designated as Hedging Instruments
Gains and losses from derivative instruments
Loss Recognized in AOCI (Effective Portion)	$ (94,660,000)	$ (155,287,000)
Loss Reclassified from AOCI into Income (Effective Portion Realized)	(117,189,000)	(122,740,000)
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)	(211,000)	3,680,000
Estimated net loss before income taxes to be reclassified from AOCI	80,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income	(52,140,000)	(106,254,000)
Not Designated as Hedging Instruments | Fuel and purchased power expense
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income	(52,113,000)	(107,690,000)
Not Designated as Hedging Instruments | Regulated electricity segment | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income	$ (27,000)	$ 1,436,000