Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
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 Loss Recognized in Income	(3,178,000)	(84,953,000)	(15,508,000)
Revenue | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Loss Recognized in Income	0	0	(2,557,000)
Fuel and purchased power | Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
Loss reclassified from accumulated other comprehensive income into income (effective portion realized)	(763,000)	(1,512,000)	(2,000,000)
Fuel and purchased power | Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Loss Recognized in Income	$ (3,178,000)	$ (84,953,000)	$ (12,951,000)