Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Designated as Hedging Instruments
Gains and losses from derivative instruments
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges	$ 0	$ 0	$ 0	$ 0
Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Loss reclassified from accumulated OCI into income (effective portion realized)	(1,459,000)	(9,772,000)	(5,232,000)	(17,426,000)
Not Designated as Hedging Instruments
Gains and losses from derivative instruments
Net gain (loss) recognized in income	(50,349,000)	(23,642,000)	(84,654,000)	(668,000)
Not Designated as Hedging Instruments | Revenue
Gains and losses from derivative instruments
Net gain (loss) recognized in income	560,000	273,000	445,000	335,000
Not Designated as Hedging Instruments | Fuel and purchased power
Gains and losses from derivative instruments
Net gain (loss) recognized in income	(50,909,000)	(23,915,000)	(85,099,000)	(1,003,000)
Other comprehensive income | Designated as Hedging Instruments
Gains and losses from derivative instruments
Gain (loss) recognized in OCI on derivative instruments (effective portion)	$ (247,000)	$ (149,000)	$ (534,000)	$ 94,000