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Nature of Operations And Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 | Jun. 17, 2023 Rate
Basis Of Accounting Presentation [Line Items] |  |  |  |
Number of customers | 3,800,000 |  |  |
Pre-tax rate for allowance for funds used during construction | 4.80% | 3.90% | 3.40% |
Inventory valued using LIFO | $ 43.8 | $ 43.9 |  |
Excess of replacement over LIFO value | (12.8) | (22.5) |  |
Inventory valued using the weighted average cost methodology | $ 135.8 | $ 222.0 |  |
NIPSCO Holdings II |  |  |  |
Basis Of Accounting Presentation [Line Items] |  |  |  |
Subsidiary, Ownership Percentage, Parent | Rate |  |  |  | 19.90%

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