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Regulatory Matters (Narrative) (Details) $ in Millions	Dec. 31, 2024 USD ($) MW	Dec. 31, 2023 USD ($)
Regulatory Assets and Liabilities Disclosure [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 671.3	$ 716.4
COVID-19 related regulatory asset	2,477.3	2,460.2
Regulatory Liabilities	1,581.7	1,789.3
Annual Revenue Increase from Joint Stipulation and Proposed Settlements	$ 28.2
Minimum
Regulatory Assets and Liabilities Disclosure [Line Items]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	15 years
Maximum
Regulatory Assets and Liabilities Disclosure [Line Items]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	75 years
Overrecovered Gas And Fuel Costs
Regulatory Assets and Liabilities Disclosure [Line Items]
Regulatory Liabilities	$ 21.4	$ 144.5
NIPSCO
Regulatory Assets and Liabilities Disclosure [Line Items]
Gas Peaker Capacity | MW	400

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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-Topic 980
-SubTopic 405
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-Section 25
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Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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