v3.25.4
Long-Term Debt (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 15, 2025
Nov. 07, 2025
Aug. 15, 2025
Jun. 27, 2025
Mar. 27, 2025
Sep. 09, 2024
Jun. 24, 2024
May 16, 2024
Mar. 14, 2024
Dec. 31, 2025
Dec. 31, 2024
Debt Instrument [Line Items]                      
Senior Notes                   $ 13,253.0 $ 12,103.0
Debt Instrument, Convertible, Conversion Ratio                   51.00%  
Security interest and other subset of asset                   $ 500.0  
Asset sale covenant percentage of consolidated total assets                   15.00%  
Cross default provision on default relating to any indebtedness                   $ 100.0  
Percentage of additional subset of assets capped                   10.00%  
Revolving credit percentage requirement in NIPSCO                   70.00%  
Junior Subordinated Notes                   $ 2,000.0 1,000.0
NiSource                      
Debt Instrument [Line Items]                      
Senior Notes       $ 900.0              
NiSource | Senior Notes                      
Debt Instrument [Line Items]                      
Senior Notes       750.0 $ 750.0         $ 1,500.0  
Proceeds from Debt, Net of Issuance Costs       $ 1,616.0 $ 739.6            
Debt, Weighted Average Interest Rate     0.95% 5.85% 5.85%            
Maturities of Senior Debt     $ 1,250.0                
NiSource | Medium-Term Note                      
Debt Instrument [Line Items]                      
Debt, Weighted Average Interest Rate                   7.99%  
NiSource | Unsecured Debt                      
Debt Instrument [Line Items]                      
Debt, Weighted Average Interest Rate       5.35%              
NiSource | 5.35% Notes Due 2034                      
Debt Instrument [Line Items]                      
Senior Notes                 $ 650.0 $ 650.0 650.0
Proceeds from Debt, Net of Issuance Costs                 $ 642.6    
Debt, Weighted Average Interest Rate                 5.35% 5.35%  
NiSource | 6.95% Notes due 2054                      
Debt Instrument [Line Items]                      
Proceeds from Debt, Net of Issuance Costs               $ 493.4      
Debt, Weighted Average Interest Rate               6.95%   6.95%  
Debt Instrument, Basis Spread on Variable Rate               2.451%      
Junior Subordinated Notes               $ 500.0   $ 500.0 500.0
NiSource | 5.20% Notes Due 2029                      
Debt Instrument [Line Items]                      
Senior Notes             $ 600.0     $ 600.0 600.0
Proceeds from Debt, Net of Issuance Costs             $ 593.7        
Debt, Weighted Average Interest Rate             5.20%     5.20%  
NiSource | 6.375% Notes due 2054                      
Debt Instrument [Line Items]                      
Proceeds from Debt, Net of Issuance Costs           $ 493.6          
Debt, Weighted Average Interest Rate           6.375%       6.375%  
Debt Instrument, Basis Spread on Variable Rate           2.527%          
Junior Subordinated Notes           $ 500.0       $ 500.0 500.0
NiSource | 5.85% Notes Due 2055                      
Debt Instrument [Line Items]                      
Senior Notes                   $ 1,500.0 0.0
Debt, Weighted Average Interest Rate                   5.85%  
NiSource | 5.35% Notes Due 2035                      
Debt Instrument [Line Items]                      
Senior Notes                   $ 900.0 0.0
Debt, Weighted Average Interest Rate                   5.35%  
NiSource | Notes due 2056                      
Debt Instrument [Line Items]                      
Proceeds from Debt, Net of Issuance Costs   $ 984.3                  
Debt, Weighted Average Interest Rate   5.75%               5.75%  
Debt Instrument, Basis Spread on Variable Rate   2.035%                  
Junior Subordinated Notes   $ 1,000.0               $ 1,000.0 $ 0.0
NIPSCO | Medium-Term Note                      
Debt Instrument [Line Items]                      
Debt, Weighted Average Interest Rate                   7.644%  
Columbia Of Massachusetts | Medium-Term Note                      
Debt Instrument [Line Items]                      
Debt, Weighted Average Interest Rate 6.43%                    
Repayments of Medium-Term Note $ 10.0                    
Maximum                      
Debt Instrument [Line Items]                      
Cross default provision on default relating to any indebtedness                   $ 50.0  
Minimum                      
Debt Instrument [Line Items]                      
Cross default provision on default relating to any indebtedness                   $ 5.0  
Revolving Credit Facility | Maximum                      
Debt Instrument [Line Items]                      
Debt Instrument, Convertible, Conversion Ratio                   70.00%