v3.26.1
Variable Interest Entities (Narrative) (Details)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Oct. 28, 2025
USD ($)
Rate
Jun. 30, 2026
USD ($)
Rate
MW
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
Rate
MW
Jun. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
Variable Interest Entity [Line Items]            
Wind Power Purchase Agreement, Purchase Percentage | Rate   100.00%   100.00%    
Income Taxes   $ 10.2 $ 23.8 $ 96.0 $ 129.5  
Contributions from NIPSCO and GenCo minority interest holders       124.5 134.3  
Payments to Noncontrolling Interests   27.1 26.9 47.0 44.4  
Business Combination, Price of Acquisition, Expected $ 35.2          
Current assets [1]   2,199.4   2,199.4   $ 2,377.2
Total Assets   37,543.6   37,543.6   35,858.7
Current liabilities [2]   4,414.5   4,414.5   3,457.4
Increase (Decrease) in Other Regulatory Assets       10.0   0.0
Variable Interest Entity Primary Beneficiary Member GenCo            
Variable Interest Entity [Line Items]            
Other Assets   210.7   210.7   305.4
us-gaap_VariableInterestEntityPrimaryBeneficiaryMemberGenCo            
Variable Interest Entity [Line Items]            
Contributions from NIPSCO and GenCo minority interest holders   39.8   47.8    
Payments to Noncontrolling Interests       0.0    
Current assets   174.3   174.3   62.5
Other Assets   213.9   213.9   305.4
Total Assets   928.3   928.3   407.6
Current liabilities   322.5   322.5   40.8
Total Liabilities   322.5   322.5   40.8
Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization   530.1   530.1   $ 39.7
NIPSCO Holdings II            
Variable Interest Entity [Line Items]            
Contributions from NIPSCO and GenCo minority interest holders   $ 51.2 $ 99.5 $ 76.7 $ 134.3  
NIPSCO Holdings II            
Variable Interest Entity [Line Items]            
Subsidiary, Ownership Percentage, Parent | Rate 19.90%          
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    
[1] Includes $1,764.0 million and $1,312.7 million at June 30, 2026 and December 31, 2025, respectively, of net property, plant and equipment assets and $223.4 million and $90.1 million at June 30, 2026 and December 31, 2025, respectively, of current assets of consolidated VIEs that may be used only to settle obligations of the consolidated VIEs. Refer to Note 4, "Noncontrolling Interests," for additional information.
(2)Includes $210.7 million and $305.4 million at June 30, 2026 and December 31, 2025, respectively, of advanced deposits of project costs and $10.0 million and zero, respectively, of regulatory assets at June 30, 2026 and December 31, 2025, respectively, of consolidated VIEs that may be used only to settle obligations of the consolidated VIEs. Refer to Note 4, "Noncontrolling Interests," for additional information.
[2] Includes $335.6 million and $56.9 million at June 30, 2026 and December 31, 2025, respectively, of current liabilities and $57.0 million and $55.7 million at June 30, 2026 and December 31, 2025, respectively, of other liabilities, and finance leases of $40.0 million and $40.1 million at June 30, 2026 and December 31, 2025, respectively, of consolidated VIEs that creditors do not have recourse to our general credit. Refer to Note 4, "Noncontrolling Interests," for additional information.