Derivative Accounting (Details 2) (Commodity Contracts, USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Designated as Hedging Instruments
Gains and losses from derivative instruments
Loss Recognized in OCI on Derivative Instruments (Effective Portion)	$ (353,000)	$ (37,663,000)	$ (94,660,000)
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized)	(44,219,000)	(99,007,000)	(117,189,000)
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)		117,000	(211,000)
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges	0	1,800,000	0
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income	21,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(10,160,000)	(2,644,000)	(52,140,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	289,000	103,000	(27,000)
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ (10,449,000)	$ (2,747,000)	$ (52,113,000)