Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Designated accounting hedging relationships
Gains and losses from derivative instruments
Amount of Loss Recognized in AOCI on Derivative Instruments (Effective Portion)	$ (16,324,000)	$ (8,588,000)	$ (15,335,000)	$ (100,255,000)
Amount of Loss Reclassified from AOCI into Income (Effective Portion Realized)	(25,287,000)	(29,143,000)	(40,133,000)	(42,329,000)
Amount of Gain (Loss) Recognized in Income from Derivative Instruments (Ineffective Portion and Amount Excluded from Effectiveness Testing)	(176,000)	11,899,000	(164,000)	1,432,000
Estimated net loss before income taxes to be reclassified from AOCI			(89,000,000)
Percentage of amounts related to derivatives subject to the PSA that will be recorded as either a regulatory asset or liability (as a percent)	90.00%		90.00%
Regulated electricity segment | Not designated as accounting hedges | Revenue
Gains and losses from derivative instruments
Amount of Net Gain (Loss) Recognized in Income from Derivative Instruments	(503,000)	426,000	1,004,000	595,000
Not designated as accounting hedges
Gains and losses from derivative instruments
Amount of Net Gain (Loss) Recognized in Income from Derivative Instruments	(3,395,000)	(28,834,000)	(10,915,000)	(63,633,000)
Not designated as accounting hedges | Fuel and purchased power expense
Gains and losses from derivative instruments
Amount of Net Gain (Loss) Recognized in Income from Derivative Instruments	$ (2,892,000)	$ (29,260,000)	$ (11,919,000)	$ (64,228,000)