Derivative Accounting (Details 2) (Commodity Contracts, USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Designated as Hedging Instruments
Gains and losses from derivative instruments
Loss recognized in OCI on derivative instruments (effective portion)	$ (240,000)	$ (119,000)	$ (409,000)	$ (37,513,000)
Loss reclassified from AOCI into income (effective portion realized)	(23,658,000)	(49,478,000)	(39,156,000)	(86,993,000)
Gain recognized in income (ineffective portion and amount excluded from effectiveness testing)				117,000
Amount reclassified from AOCI to earnings related to discontinued cash flow hedges	0	0	0	1,800,000
Estimated net loss before income taxes to be reclassified from AOCI			23,000,000
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net gain recognized in income from Derivative Instruments	(1,145,000)	13,128,000	(11,350,000)	13,879,000
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net gain recognized in income from Derivative Instruments	196,000	258,000	400,000	19,000
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net gain recognized in income from Derivative Instruments	$ (1,341,000)	$ 12,870,000	$ (11,750,000)	$ 13,860,000