v3.26.1
Revenue Recognition (Disaggregation of Revenue) (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
Disaggregation of Revenue [Line Items]          
Customer revenues $ 1,289.5 $ 1,246.5 $ 3,611.8 $ 3,396.0  
Other revenues 52.9 36.5 93.7 70.2  
Total Operating Revenues 1,342.4 1,283.0 3,705.5 3,466.2  
Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 1,344.7 1,285.5 3,710.1 3,471.0  
Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 1,340.9 1,281.8 3,702.6 3,463.8  
NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues       947.6  
NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 752.4 777.9 2,552.4 2,448.4  
NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 537.1 468.6 1,059.4    
Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Other revenues 1.5 1.2 2.9 2.4  
Corporate and Other | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues 1.5 1.2 2.9 2.4  
NIPSCO Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 704.7 654.2   1,563.8  
Other revenues 39.2 26.4 84.0 58.2  
Total Operating Revenues 743.9 680.6      
NIPSCO Operations | Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 744.2 680.8 1,783.0 1,622.5  
NIPSCO Operations | Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 743.9 680.6 1,782.4    
NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 167.6 185.6 639.0 616.2  
NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 537.1 468.6   947.6  
NIPSCO Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues       1,622.0  
Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 584.8 592.3 1,913.4 1,832.2  
Other revenues 12.2 8.9 6.8 9.6  
Columbia Operations | Operating Segments          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 600.5 604.7 1,927.1 1,848.5  
Columbia Operations | Operating Segments | Nonrelated Party          
Disaggregation of Revenue [Line Items]          
Total Operating Revenues 597.0 601.2 1,920.2    
Columbia Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues     1,920.2 1,841.8  
NIPSCO          
Disaggregation of Revenue [Line Items]          
Customer revenues     1,698.4    
Total Operating Revenues     1,782.4    
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Abstract]          
Contract with Customer, Liability 95.0   95.0   $ 0.0
NIPSCO | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues     1,059.4    
Residential | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 502.1 511.2 1,729.9 1,653.8  
Residential | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 183.1 163.9 359.9 331.8  
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 102.2 114.2 420.8 405.6  
Residential | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 183.1 163.9 359.9 331.8  
Residential | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 399.9 397.0 1,309.1 1,248.2  
Commercial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 166.0 171.7 612.6 581.3  
Commercial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 181.7 161.0 353.8 321.1  
Commercial | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues 0.0 0.0 0.0 0.0  
Commercial | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 41.4 45.5 157.0 148.6  
Commercial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 181.7 161.0 353.8 321.1  
Commercial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 124.6 126.2 455.6 432.7  
Industrial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 61.3 60.2 152.8 139.5  
Industrial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 159.2 133.5 314.7 276.1  
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 21.0 22.6 55.1 54.0  
Industrial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 159.2 133.5 314.7 276.1  
Industrial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 40.3 37.6 97.7 85.5  
Off-system | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 11.5 22.8 28.3 45.1  
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 11.5 22.8 28.3 45.1  
Miscellaneous | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues 11.5 12.0 28.8 28.7  
Miscellaneous | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues (0.5) (1.7) 4.6    
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 3.0 3.3 6.1 8.0  
Miscellaneous | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues (0.5) (1.7) 4.6 (0.9)  
Miscellaneous | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues 8.5 8.7 22.7 20.7  
Wholesale | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues 9.9 11.9 18.4 19.5  
Wholesale | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues 9.9 $ 11.9 18.4 19.5  
Public Authority          
Disaggregation of Revenue [Line Items]          
Customer revenues 3.7   8.0 $ (0.9)  
Public Authority | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues $ 3.7   $ 8.0