Derivative Accounting - Gains and Losses from Derivative Instruments (Details) (Commodity Contracts, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
Loss Recognized in OCI on Derivative Instruments (Effective Portion)	$ (372,000)	$ (353,000)	$ (37,663,000)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion Realized)	(21,415,000)	(44,219,000)	(99,007,000)
Gain Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	0	117,000
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges	0	0	1,800,000
Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	(66,043,000)	(10,160,000)	(2,644,000)
Revenue | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	324,000	289,000	103,000
Fuel and purchased power | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	$ (66,367,000)	$ (10,449,000)	$ (2,747,000)