Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Customer revenues	$ 2,322.3	$ 2,149.5
Other revenues	40.8	33.7
Total Operating Revenues	2,363.1	2,183.2
Operating Segments
Disaggregation of Revenue [Line Items]
Total Operating Revenues	2,365.4	2,185.5
Operating Segments \| Nonrelated Party
Disaggregation of Revenue [Line Items]
Total Operating Revenues	2,361.7	2,182.0
NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	1,800.0	1,670.5
NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	522.3	479.0
Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0
Other revenues	1.4	1.2
Corporate and Other \| Unaffiliated
Disaggregation of Revenue [Line Items]
Revenues	1.4	1.2
NIPSCO Operations
Disaggregation of Revenue [Line Items]
Customer revenues	993.7	909.6
Other revenues	44.8	31.8
Total Operating Revenues	1,038.5	941.4
NIPSCO Operations \| Operating Segments
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,038.8	941.7
NIPSCO Operations \| Operating Segments \| Nonrelated Party
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,038.5	941.4
NIPSCO Operations \| Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	471.4	430.6
NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	522.3	479.0
Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues	1,328.6	1,239.9
Other revenues	(5.4)	0.7
Columbia Operations \| Operating Segments
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,326.6	1,243.8
Columbia Operations \| Operating Segments \| Nonrelated Party
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,323.2	1,240.6
Residential \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	1,227.8	1,142.6
Residential \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	176.8	167.9
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	318.6	291.4
Residential \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	176.8	167.9
Residential \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues	909.2	851.2
Commercial \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	446.6	409.6
Commercial \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	172.1	160.1
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	115.6	103.1
Commercial \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	172.1	160.1
Commercial \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues	331.0	306.5
Industrial \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	91.5	79.3
Industrial \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	155.5	142.6
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	34.1	31.4
Industrial \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	155.5	142.6
Industrial \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues	57.4	47.9
Off-system \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	16.8	22.3
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	16.8	22.3
Miscellaneous \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues	17.3	15.6
Miscellaneous \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	5.1	(1.4)
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	3.1	4.7
Miscellaneous \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	5.1	(1.4)
Miscellaneous \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues	14.2	10.9
Wholesale \| NiSource Gas Distribution
Disaggregation of Revenue [Line Items]
Customer revenues		1.1
Wholesale \| NiSource Electric Generation and Power Delivery
Disaggregation of Revenue [Line Items]
Customer revenues	8.5	7.6
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	8.5	7.6
Wholesale \| Columbia Operations
Disaggregation of Revenue [Line Items]
Customer revenues		1.1
Public Authority
Disaggregation of Revenue [Line Items]
Customer revenues	4.3	2.2
Public Authority \| NIPSCO Operations \| NIPSCO Electric Generation
Disaggregation of Revenue [Line Items]
Customer revenues	$ 4.3	$ 2.2

NISOURCE INC. 10-Q