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Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($)
 3 Months Ended 9 Months Ended
Jun. 12, 2025
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Disaggregation of Revenue [Line Items]          
Customer revenues   $ 1,240,200,000 $ 1,046,100,000 $ 4,636,200,000 $ 3,743,200,000
Other revenues   32,900,000 30,200,000 103,100,000 124,100,000
Total Operating Revenues   1,273,100,000 1,076,300,000 4,739,300,000 3,867,300,000
NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   607,100,000 531,000,000.0 3,055,500,000 2,365,200,000
NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   633,100,000 515,100,000 1,580,700,000 1,378,000,000
Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues $ 2,518,393 0 0 0 0
Other revenues   1,200,000 300,000 3,600,000 600,000
Corporate and Other | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues   1,200,000 300,000 3,600,000 600,000
NIPSCO Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   760,500,000 628,800,000 2,324,300,000 1,938,300,000
Other revenues   26,500,000 23,800,000 84,700,000 63,900,000
Total Operating Revenues   787,000,000.0 652,600,000    
NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   127,400,000 113,700,000 743,600,000 560,300,000
NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   633,100,000 515,100,000 1,580,700,000 1,378,000,000
NIPSCO Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues       2,409,000,000 2,002,200,000
Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   479,700,000 417,300,000 2,311,900,000 1,804,900,000
Other revenues   5,200,000 6,100,000 14,800,000 59,600,000
Columbia Operations | Unaffiliated          
Disaggregation of Revenue [Line Items]          
Revenues       2,326,700,000 1,864,500,000
Residential | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   401,000,000.0 363,700,000 2,054,800,000 1,614,700,000
Residential | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   254,600,000 197,900,000 586,400,000 498,600,000
Residential | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Residential | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   72,900,000 66,800,000 478,500,000 357,500,000
Residential | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   254,600,000 197,900,000 586,400,000 498,600,000
Residential | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   328,100,000 296,900,000 1,576,300,000 1,257,200,000
Commercial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   127,300,000 106,900,000 708,600,000 529,600,000
Commercial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   210,600,000 172,700,000 531,700,000 470,000,000.0
Commercial | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Commercial | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   32,200,000 28,600,000 180,800,000 134,000,000.0
Commercial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   210,600,000 172,700,000 531,700,000 470,000,000.0
Commercial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   95,100,000 78,300,000 527,800,000 395,600,000
Industrial | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   55,700,000 48,400,000 195,200,000 161,400,000
Industrial | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   156,200,000 124,800,000 432,300,000 360,400,000
Industrial | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Industrial | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   19,700,000 16,400,000 73,700,000 56,300,000
Industrial | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   156,200,000 124,800,000 432,300,000 360,400,000
Industrial | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   36,000,000.0 32,000,000.0 121,500,000 105,100,000
Off-system | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   13,800,000 7,100,000 58,900,000 30,500,000
Off-system | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Off-system | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Off-system | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   13,800,000 7,100,000 58,900,000 30,500,000
Miscellaneous | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   9,300,000 4,800,000 38,000,000.0 27,900,000
Miscellaneous | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   (6,700,000) 2,700,000 (7,600,000) 10,600,000
Miscellaneous | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues   0 0 0 0
Miscellaneous | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues   2,600,000 1,900,000 10,600,000 12,500,000
Miscellaneous | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   (6,700,000) 2,700,000 (7,600,000) 10,600,000
Miscellaneous | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues   6,700,000 2,900,000 27,400,000 15,400,000
Wholesale | NiSource Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues     100,000   1,100,000
Wholesale | NiSource Electric Generation and Power Delivery          
Disaggregation of Revenue [Line Items]          
Customer revenues   18,400,000 15,000,000.0 37,900,000 32,400,000
Wholesale | Corporate and Other          
Disaggregation of Revenue [Line Items]          
Customer revenues     0   0
Wholesale | NIPSCO Operations | Gas Distribution          
Disaggregation of Revenue [Line Items]          
Customer revenues     0   0
Wholesale | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues   $ 18,400,000 15,000,000.0 $ 37,900,000 32,400,000
Wholesale | Columbia Operations          
Disaggregation of Revenue [Line Items]          
Customer revenues     100,000   1,100,000
Public Authority          
Disaggregation of Revenue [Line Items]          
Customer revenues     2,000,000.0   6,000,000.0
Public Authority | NIPSCO Operations | NIPSCO Electric Generation          
Disaggregation of Revenue [Line Items]          
Customer revenues     $ 2,000,000.0   $ 6,000,000.0