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Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Derivative [Line Items] |  |  |
Regulatory assets | $ 802,257 |  | $ 278,258
Gas price derivatives |  |  |
Derivative [Line Items] |  |  |
Regulatory assets | 324 |  | $ 2,226
Not Designated as Hedging Instrument |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | (1,158) | $ 2,128 |
Not Designated as Hedging Instrument | Fuel, purchased power and cost of natural gas sold | Electricity |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | (1,524) | 1,362 |
Not Designated as Hedging Instrument | Fuel, purchased power and cost of natural gas sold | Natural Gas, Distribution |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ 366 | $ 766 |

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