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Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Derivative [Line Items] |  |  |  |  |
Regulatory assets | $ 391.2 |  | $ 391.2 |  | $ 427.7
Regulatory liabilities | 576.6 |  | 576.6 |  | 568.7
Gas price derivatives |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Regulatory assets | 4.9 |  | 4.9 |  | $ 2.9
Not Designated as Hedging Instrument |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ (0.6) | $ (0.3) | $ 0.0 | $ 0.4 |
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold |
Not Designated as Hedging Instrument | Natural Gas |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ (0.6) | $ (0.3) | $ 0.0 | $ 0.4 |
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold |

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