| **Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)**   | 3 Months Ended |
|----------------------------------------------------------------------------------------------------------------------|----------------|
||
| 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 |
| Designated as Hedging Instruments | Fuel and purchased power                                                         |           |   |
| **Gains and losses from derivative instruments**                                                                     |           |   |
| Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized)                                      | (436,000)      | (491,000) |
| Not Designated as Hedging Instruments                                                                                |           |   |
| **Gains and losses from derivative instruments**                                                                     |           |   |
| Net Gain (Loss) Recognized in Income                                                                                 | 8,170,000      | (35,308,000) |
| Not Designated as Hedging Instruments | Operating revenues                                                           |           |   |
| **Gains and losses from derivative instruments**                                                                     |           |   |
| Net Gain (Loss) Recognized in Income                                                                                 | 0              | (1,219,000) |
| Not Designated as Hedging Instruments | Fuel and purchased power                                                     |           |   |
| **Gains and losses from derivative instruments**                                                                     |           |   |
| Net Gain (Loss) Recognized in Income                                                                                 | $ 8,170,000    | $ (34,089,000) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net gain (loss) reclassified from accumulated other comprehensive income into income on discontinuation of derivative instruments designated and qualifying as hedging instruments.
+ References
No definition available.
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| X                                                                                                                                                                                                                   |
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| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.
+ References
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| X                                                                                                                                                            |
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| X                                                                                                                                                     |
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| X                                                                                                                                                  |
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| X                                                                                                                                               |
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