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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Interest expense | $ 180.0 | $ 162.6 | $ 154.1
Fuel, purchased power and cost of natural gas sold | 982.9 | 1,230.6 | 741.9
Operations and maintenance | 552.0 | 548.4 | 501.7
Income Tax Expense (Benefit) | (25.6) | (25.2) | (7.2)
Net income | 276.0 | 270.8 | $ 251.2
Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Operations and maintenance | 0.0 | 0.1 |
Net income | (4.7) | (0.6) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Income before income taxes | (5.9) | (0.1) |
Income Tax Expense (Benefit) | 1.4 | 0.0 |
Net income | (4.5) | (0.1) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Swap |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Interest expense | (2.9) | (2.8) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Fuel, purchased power and cost of natural gas sold | (3.0) | 2.7 |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Operations and maintenance | (0.2) | (0.8) |
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Income before income taxes | (0.2) | (0.7) |
Income Tax Expense (Benefit) | 0.0 | 0.2 |
Net income | $ (0.2) | $ (0.5) |

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