Derivative Accounting (Details 2) (Commodity Contracts, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Designated as Hedging Instruments
Gains and losses from derivative instruments
Gain Recognized in OCI on Derivative Instruments (Effective Portion)	$ 177	$ 96
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized)	(4,439)	(8,353)
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges	0	0
Estimated net loss before income taxes to be reclassified from accumulated other comprehensive income	(19,000)
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	18,015	17,233
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	(92)	(117)
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net Gain (Loss) Recognized in Income from Derivative Instruments	$ 18,107	$ 17,350