v3.25.4
Income Taxes (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Oct. 28, 2025
Income Taxes [Line Items]        
Deferred Tax Assets, Operating Loss Carryforwards $ 326.0 $ 369.4    
Deferred Tax Assets, Valuation Allowance 14.8 6.4    
Book income before income taxes 1,216.4 1,002.8 $ 813.9  
Tax expense (benefit) at statutory federal income tax rate, value $ 255.4 $ 210.6 $ 170.8  
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 21.00%  
State income taxes, net of federal income tax benefit, value $ 4.9 $ 11.7 $ 13.7  
State income taxes, net of federal income tax benefit, rate 0.40% 1.20% 1.70%  
Fines and penalties, value $ 5.5 $ 12.5 $ 5.3  
Fines and penalties, rate 0.40% 1.20% 0.70%  
State regulatory proceedings, rate 0.10% (0.80%) (0.90%)  
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent (1.50%) (2.70%) 0.00%  
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount $ (18.2) $ (27.3) $ 0.0  
Other adjustments, value $ (4.4) $ 2.5 $ (4.4)  
Other adjustments, rate (0.30%) (0.20%) 0.50%  
Income Taxes $ 203.8 $ 158.1 $ 139.5  
Effective Income Tax Rate Reconciliation, Percent 16.80% 15.80% 17.10%  
Deferred Federal Income Tax Expense (Benefit) $ (411.4)      
Investment Tax Credit 32.4 $ 14.5 $ 2.1  
Increase (Decrease) in Tax Expense 45.7 18.6    
Operating Loss Carryforwards 21.7 21.7 21.7  
Investment and Production Tax Credits 19.3 18.8    
Prior Period Federal Tax Credit Passback $ 4.6      
NiSource        
Income Taxes [Line Items]        
Subsidiary, Ownership Percentage, Parent 80.10%     80.10%
Investment Tax Credit Carryforward        
Income Taxes [Line Items]        
Deferred Tax Assets, Valuation Allowance $ 0.0      
Deferred Federal Income Tax Expense (Benefit) (31.0)      
Investment Tax Credit 0.0 $ 0.1 $ 2.1  
Federal production tax credits        
Income Taxes [Line Items]        
Deferred Tax Assets, Valuation Allowance 0.0      
Deferred Federal Income Tax Expense (Benefit) (23.4)      
Federal other credit        
Income Taxes [Line Items]        
Deferred Tax Assets, Valuation Allowance 0.0      
Deferred Federal Income Tax Expense (Benefit) (22.9)      
State and Local Jurisdiction        
Income Taxes [Line Items]        
Deferred Federal, State and Local, Tax Expense (Benefit) 75.4      
Domestic Tax Jurisdiction        
Income Taxes [Line Items]        
Deferred Federal Income Tax Expense (Benefit) (250.3)      
Federal disallowed interest expense carryforward        
Income Taxes [Line Items]        
Interest Expense, Federal Funds Purchased, Increase (Decrease) (8.4)      
State and Local Jurisdiction        
Income Taxes [Line Items]        
Deferred Tax Assets, Valuation Allowance 6.4      
Operating Loss Carryforwards 1,867.1      
Domestic Tax Jurisdiction        
Income Taxes [Line Items]        
Deferred Tax Assets, Valuation Allowance 0.0      
Operating Loss Carryforwards $ 1,192.0