v3.26.3
Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Disaggregation of Revenue [Line Items]          
Customer revenues $ 1,299.1 $ 1,240.2 $ 4,910.9 $ 4,636.2  
Other revenues 50.3 32.9 144.0 103.1  
Total Operating Revenues 1,349.4 1,273.1 5,054.9 4,739.3  
Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 1,341.1 1,275.5 5,051.2 4,746.5  
Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 1,337.4 1,271.9 5,040.0 4,735.7  
NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues       1,580.7  
NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 625.9 607.1 3,178.3 3,055.5  
NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 673.2 633.1 1,732.6    
Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Other revenues 1.5 1.2 4.4 3.6  
Corporate and Other | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 10.5 0.0 10.5    
Corporate and Other | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues 12.0 1.2 14.9 3.6  
NIPSCO Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 795.7 760.5   2,324.3  
Other revenues 39.5 26.5 123.5 84.7  
Total Operating Revenues 835.2 787.0      
NIPSCO Operations | Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 835.5 787.3 2,618.5 2,409.8  
NIPSCO Operations | Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 835.2 787.0 2,617.6    
NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 133.0 127.4 772.0 743.6  
NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 662.7 633.1   1,580.7  
NIPSCO Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues       2,409.0  
Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 492.9 479.7 2,406.3 2,311.9  
Other revenues 9.3 5.2 16.1 14.8  
Columbia Operations | Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 505.6 488.2 2,432.7 2,336.7  
Columbia Operations | Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 502.2 484.9 2,422.4    
Columbia Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues     2,422.4 2,326.7  
NIPSCO          
Disaggregation of Revenue [Line Items]          
Customer revenues 1.6   2,494.1    
Total Operating Revenues     2,617.6    
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Abstract]          
Contract with Customer, Liability 113.4   113.4   $ 0.0
NIPSCO | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues     1,722.1    
Corporate Segment          
Disaggregation of Revenue [Line Items]          
Customer revenues 10.5   10.5    
Residential | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 418.5 401.0 2,148.4 2,054.8  
Residential | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 249.4 254.6 609.3 586.4  
Residential | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Residential | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 75.1 72.9 495.9 478.5  
Residential | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 249.4 254.6 609.3 586.4  
Residential | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 343.4 328.1 1,652.5 1,576.3  
Residential | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0      
Commercial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 133.6 127.3 746.2 708.6  
Commercial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 219.9 210.6 573.7 531.7  
Commercial | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0   0.0  
Commercial | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 35.4 32.2 192.4 180.8  
Commercial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 213.4 210.6 567.2 531.7  
Commercial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 98.2 95.1 553.8 527.8  
Commercial | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 6.5 0.0 6.5 0.0  
Industrial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 55.9 55.7 208.7 195.2  
Industrial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 178.8 156.2 493.5 432.3  
Industrial | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Industrial | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 19.9 19.7 75.0 73.7  
Industrial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 178.8 156.2 493.5 432.3  
Industrial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 36.0 36.0 133.7 121.5  
Industrial | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0      
Off-system | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 6.8 13.8 35.1 58.9  
Off-system | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Off-system | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Off-system | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 6.8 13.8 35.1 58.9  
Miscellaneous | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 11.1 9.3 39.9 38.0  
Miscellaneous | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 1.3 (6.7) 5.9    
Miscellaneous | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Miscellaneous | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 2.6 2.6 8.7 10.6  
Miscellaneous | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 1.3 (6.7) 5.9 (7.6)  
Miscellaneous | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 8.5 6.7 31.2 27.4  
Miscellaneous | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0      
Wholesale | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 19.7 18.4 38.1 37.9  
Wholesale | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 15.7 18.4 34.1 37.9  
Wholesale | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 4.0 $ 0.0 4.0 0.0  
Public Authority          
Disaggregation of Revenue [Line Items]          
Customer revenues 4.1   12.1 $ (7.6)  
Public Authority | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues     0.0    
Public Authority | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 4.1   $ 12.1    
Public Authority | GenCo Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues $ 0.0