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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, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023
Derivative [Line Items] |  |  |  |  |
Regulatory assets | $ 432.2 |  | $ 432.2 |  | $ 480.1
Regulatory liabilities | 557.0 |  | 557.0 |  | 566.6
Gas price derivatives |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Regulatory assets | 3.2 |  | 3.2 |  | $ 5.1
Not Designated as Hedging Instrument |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ (0.3) | $ 0.3 | $ 0.4 | $ (2.4) |
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 | $ (0.3) | $ 0.3 | $ 0.4 | $ (2.4) |
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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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A + Details
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- Definition Indicates line item in statement of income or comprehensive income that includes gain (loss) from derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A + Details
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- 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480237/815-40-50-5 + Details
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