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Regulatory Matters (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2024 USD ($) MW | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) MW | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Depreciation, Nonproduction | $ (24.2) | $ (9.7) | $ (40.8) | $ (19.0) |
Deferred Derivative Gain (Loss) |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Regulatory Assets | 11.8 |  | 11.8 |  | $ 24.4
Deferred Derivative Gain (Loss) | NIPSCO |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Regulatory Assets | $ 16.9 |  | $ 16.9 |  |
Rosewater |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Nameplate Capacity | MW | 102 |  | 102 |  |
NIPSCO |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Nameplate Capacity | MW | 400 |  | 400 |  |

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