Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($)		3 Months Ended	12 Months Ended
	Jun. 12, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Customer revenues		$ 6,522,800,000	$ 6,522,800,000	$ 5,282,900,000	$ 5,347,800,000
Other revenues		119,400,000	119,400,000	172,200,000	157,600,000
Total Operating Revenues		6,642,200,000	6,642,200,000	5,455,100,000	5,505,400,000
NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		2,106,300,000		1,829,500,000	1,701,000,000
NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			4,416,500,000	3,453,400,000	3,646,800,000
Electric Operations
Disaggregation of Revenue [Line Items]
Total Operating Revenues				2,751,000,000	2,770,700,000
Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues	$ 2,518,393		0	0	0
Other revenues			4,900,000	900,000	800,000
Total Operating Revenues			4,900,000	900,000	800,000
NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			3,198,000,000	2,654,500,000	2,686,800,000
Other revenues			109,300,000	96,500,000	83,900,000
Total Operating Revenues		3,307,300,000
NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			2,106,300,000	1,829,500,000	1,701,000,000
NIPSCO Operations \| NIPSCO Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			1,091,700,000	825,000,000.0	985,800,000
Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			3,324,800,000	2,628,400,000	2,661,000,000
Other revenues			5,200,000	74,800,000	72,900,000
Total Operating Revenues			3,330,000,000	2,703,200,000	2,733,900,000
Residential \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		768,400,000		649,900,000	583,900,000
Residential \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			2,987,900,000	2,365,200,000	2,462,500,000
Residential \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				531,400,000	635,000,000.0
Residential \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Residential \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			708,000,000.0
Residential \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			768,400,000	649,900,000	583,900,000
Residential \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			2,279,900,000	1,833,800,000	1,827,500,000
Commercial \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		713,900,000		620,400,000	578,100,000
Commercial \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			1,033,700,000	779,500,000	847,900,000
Commercial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				199,200,000	249,000,000.0
Commercial \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Commercial \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			270,000,000.0
Commercial \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			713,900,000	620,400,000	578,100,000
Commercial \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			763,700,000	580,300,000	598,900,000
Industrial \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		578,300,000		499,100,000	474,100,000
Industrial \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			267,600,000	223,200,000	226,000,000.0
Industrial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				79,000,000.0	87,000,000.0
Industrial \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Industrial \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			100,200,000
Industrial \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			578,300,000	499,100,000	474,100,000
Industrial \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			167,400,000	144,200,000	139,000,000.0
Off-system \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			75,800,000	42,600,000	60,600,000
Off-system \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				0	0
Off-system \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Off-system \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			0
Off-system \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			75,800,000	42,600,000	60,600,000
Miscellaneous \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		(8,900,000)		13,700,000	21,400,000
Miscellaneous \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			49,400,000	41,700,000	48,200,000
Miscellaneous \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				15,400,000	14,800,000
Miscellaneous \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Miscellaneous \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			13,500,000
Miscellaneous \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			(8,900,000)	13,700,000	21,400,000
Miscellaneous \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			35,900,000	26,300,000	33,400,000
Wholesale \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues		45,100,000		38,300,000	32,000,000.0
Wholesale \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues			2,100,000	1,200,000	1,600,000
Wholesale \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues				0	0
Wholesale \| Corporate Segment and Other Operating Segment
Disaggregation of Revenue [Line Items]
Customer revenues			0	0	0
Wholesale \| NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues			0
Wholesale \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			45,100,000	38,300,000	32,000,000.0
Wholesale \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues			2,100,000	1,200,000	1,600,000
Public Authority
Disaggregation of Revenue [Line Items]
Customer revenues		$ 9,500,000		8,100,000	11,500,000
Public Authority \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues			$ 9,500,000	$ 8,100,000	$ 11,500,000