Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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 Gain (Loss) Recognized in Income	(84,953,000)	(15,508,000)	(89,183,000)
Revenue \| Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	0	(2,557,000)	(1,192,000)
Fuel and purchased power \| Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion Realized)	(1,512,000)	(2,000,000)	(3,519,000)
Fuel and purchased power \| Not Designated as Hedging Instruments
Derivative Instruments Not Designated as Cash Flows Hedges
Net Gain (Loss) Recognized in Income	(84,953,000)	(12,951,000)	(87,991,000)
Other Comprehensive Income (Loss) \| Designated as Hedging Instruments
Derivative Instruments in Designated Cash Flows Hedges
Loss Recognized in OCI on Derivative Instruments (Effective Portion)	$ 0	$ 0	$ (59,000)

The net gain (loss) reclassified from accumulated other comprehensive income into income on discontinuation of derivative instruments designated and qualifying as hedging instruments.

No definition available.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

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- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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