v3.26.1
Business Segment Information (Schedule Of Operating Income Derived From Revenues And Expenses By Segment) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Segment Reporting [Line Items]          
Consolidated Operating Income (Loss) $ 228.1 $ 262.9 $ 1,047.3 $ 1,022.3  
Total Assets 37,543.6   37,543.6   $ 35,858.7
Regulated Operating Revenue 1,342.4 1,283.0 3,705.5 3,466.2  
Cost of energy 193.6 261.8 862.8 909.3  
Utilities Operating Expense, Maintenance and Operations 474.9 396.2 964.1 824.0  
Taxes, Miscellaneous 83.6 74.2 180.8 163.8  
Depreciation, Depletion and Amortization 362.2 286.7 650.5 545.3  
Operating Segments          
Segment Reporting [Line Items]          
Consolidated Operating Income (Loss) 223.7 256.2 1,044.5 1,013.9  
Regulated Operating Revenue 1,344.7 1,285.5 3,710.1 3,471.0  
Cost of energy 192.6 261.8 861.8 909.3  
Utilities Operating Expense, Maintenance and Operations 495.6 418.7 995.4 863.7  
Taxes, Miscellaneous 79.2 70.5 171.2 156.0  
Segment Reporting, Other Segment Item, Amount   0.7   1.0  
Depreciation, Depletion and Amortization 353.6 277.6 637.2 527.1  
Operating Segments | Nonrelated Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue 1,340.9 1,281.8 3,702.6 3,463.8  
Operating Segments | Related Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue 3.8 3.7 7.5 7.2  
Corporate and Other          
Segment Reporting [Line Items]          
Consolidated Operating Income (Loss) 4.4 6.7 2.8 8.4  
Total Assets 2,218.7   2,218.7   1,828.3
Regulated Operating Revenue 163.4 144.6 310.7 290.0  
Eliminations          
Segment Reporting [Line Items]          
Regulated Operating Revenue (165.7) (147.1) (315.3) (294.8)  
Corporate and Other | Unaffiliated          
Segment Reporting [Line Items]          
Revenues 1.5 1.2 2.9 2.4  
Columbia Operations | Unaffiliated          
Segment Reporting [Line Items]          
Revenues     1,920.2 1,841.8  
Columbia Operations | Intersegment          
Segment Reporting [Line Items]          
Revenues       6.7  
Columbia Operations | Operating Segments          
Segment Reporting [Line Items]          
Consolidated Operating Income (Loss) 103.3 122.2 575.6 568.0  
Total Assets 16,258.5   16,258.5   15,903.7
Regulated Operating Revenue 600.5 604.7 1,927.1 1,848.5  
Cost of energy 80.8 110.4 464.7 490.2  
Utilities Operating Expense, Maintenance and Operations 225.2 207.7 503.6 450.7  
Taxes, Miscellaneous 59.8 52.4 130.9 119.4  
Segment Reporting, Other Segment Item, Amount   0.3   0.3  
Depreciation, Depletion and Amortization 131.4 111.7 252.3 219.9  
Columbia Operations | Operating Segments | Nonrelated Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue 597.0 601.2 1,920.2    
Columbia Operations | Operating Segments | Related Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue 3.5 3.5 6.9    
NIPSCO Operations          
Segment Reporting [Line Items]          
Regulated Operating Revenue 743.9 680.6      
NIPSCO Operations | Unaffiliated          
Segment Reporting [Line Items]          
Revenues       1,622.0  
NIPSCO Operations | Intersegment          
Segment Reporting [Line Items]          
Revenues       0.5  
NIPSCO Operations | Operating Segments          
Segment Reporting [Line Items]          
Consolidated Operating Income (Loss) 120.4 134.0 468.9 445.9  
Total Assets 19,066.4   19,066.4   $ 18,126.7
Regulated Operating Revenue 744.2 680.8 1,783.0 1,622.5  
Cost of energy 111.8 151.4 397.1 419.1  
Utilities Operating Expense, Maintenance and Operations 270.4 211.0 491.8 413.0  
Taxes, Miscellaneous 19.4 18.1 40.3 36.6  
Segment Reporting, Other Segment Item, Amount   0.4   0.7  
Depreciation, Depletion and Amortization 222.2 165.9 384.9 $ 307.2  
NIPSCO Operations | Operating Segments | Nonrelated Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue 743.9 680.6 1,782.4    
NIPSCO Operations | Operating Segments | Related Party          
Segment Reporting [Line Items]          
Regulated Operating Revenue $ 0.3 $ 0.2 $ 0.6