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Income Taxes (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Income Taxes [Line Items]      
Deferred Tax Assets, Operating Loss Carryforwards $ 369.4 $ 422.9  
Deferred Tax Assets, Valuation Allowance 6.4 6.4  
Book income before income taxes 1,002.8 813.9 $ 956.4
Tax expense (benefit) at statutory federal income tax rate, value $ 210.6 $ 170.8 $ 200.8
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 $ 11.7 $ 13.7 $ 4.5
State income taxes, net of federal income tax benefit, rate 1.20% 1.70% 0.50%
Amortization of regulatory liabilities, value $ (11.1) $ (38.2) $ (38.5)
Amortization of regulatory liabilities, rate (1.10%) (4.70%) (4.00%)
Effective Income Tax Rate Reconciliation, Deduction, Employee Stock Ownership Plan Dividend, Amount $ (1.3) $ (1.3) $ (1.2)
Employee stock ownership plan dividends and other compensation, rate (0.10%) (0.20%) (0.10%)
Effective Income Tax Rate Reconciliation, Tax Credit, Amount $ (23.3) $ (4.9) $ (2.3)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (2.30%) (0.60%) (0.20%)
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent (2.70%)    
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount $ (27.3) $ 0.0  
Other adjustments, value $ (1.2) $ (0.6) $ 1.3
Other adjustments, rate (0.20%) 0.10% 0.00%
Income Taxes $ 158.1 $ 139.5 $ 164.6
Effective Income Tax Rate Reconciliation, Percent 15.80% 17.10% 17.20%
Deferred Federal Income Tax Expense (Benefit) $ (389.9)    
Increase (Decrease) in Tax Expense 18.6 $ (25.1)  
Operating Loss Carryforwards 21.7 21.7 $ 21.7
Decrease in Amortization of Regulatory Liabilities $ 7.9    
NiSource      
Income Taxes [Line Items]      
Subsidiary, Ownership Percentage, Parent 80.10%    
Investment Tax Credit Carryforward      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance $ 0.0    
Deferred Federal Income Tax Expense (Benefit) (2.2)    
Investment Tax Credit 0.1 $ 2.1 $ 0.0
Federal production tax credits      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance 0.0    
Deferred Federal Income Tax Expense (Benefit) (1.0)    
Federal other credit      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance 0.0    
Deferred Federal Income Tax Expense (Benefit) (17.3)    
State and Local Jurisdiction      
Income Taxes [Line Items]      
Deferred Federal, State and Local, Tax Expense (Benefit) 81.9    
Domestic Tax Jurisdiction      
Income Taxes [Line Items]      
Deferred Federal Income Tax Expense (Benefit) (287.5)    
State and Local Jurisdiction      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance 6.4    
Operating Loss Carryforwards 2,024.4    
Domestic Tax Jurisdiction      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance 0.0    
Operating Loss Carryforwards $ 1,369.1