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Variable Interest Entities (Narrative) (Details) $ in Millions					3 Months Ended			9 Months Ended
	Jun. 18, 2025 USD ($)	May 31, 2025 USD ($)	Feb. 03, 2025 USD ($)	Jan. 15, 2025 USD ($)	Sep. 30, 2025 USD ($) Rate MW	Mar. 31, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) Rate MW	Sep. 30, 2024 USD ($)
Variable Interest Entity [Line Items]
Wind Power Purchase Agreement, Purchase Percentage | Rate					100.00%			100.00%
Payments to Acquire Assets, Investing Activities | $	$ 262.4	$ 141.4	$ 217.6	$ 336.6
Income Taxes | $					$ 20.6		$ 15.9	$ 150.1	$ 109.5
Contributions from NIPSCO minority interest holders | $					11.0		39.8	145.3	99.5
Payments to Noncontrolling Interests | $					$ 11.1	$ 32.0	$ 11.8	$ 55.5	$ 32.0
Rosewater
Variable Interest Entity [Line Items]
Nameplate Capacity | MW					102			102
Indiana Crossroads Wind
Variable Interest Entity [Line Items]
Nameplate Capacity | MW					302			302
Indiana Crossroads Solar
Variable Interest Entity [Line Items]
Nameplate Capacity | MW					200			200
Dunns Bridge I
Variable Interest Entity [Line Items]
Nameplate Capacity | MW					265			265

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