Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion)	$ (59,000)	$ 47,000	$ (615,000)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion Realized)	(3,519,000)	(3,926,000)	(5,988,000)
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges	0	0	0
Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	(89,183,000)	26,482,000	(108,399,000)
Revenue | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	(1,192,000)	771,000	574,000
Fuel and purchased power | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	$ (87,991,000)	$ 25,711,000	$ (108,973,000)