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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