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Long-Term Debt (Schedule of Consolidated Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 15, 2025	Nov. 07, 2025	Jun. 27, 2025	Dec. 31, 2024	Sep. 09, 2024	Jun. 24, 2024	May 16, 2024	Mar. 14, 2024
Debt Instrument [Line Items]
Medium-Term Notes	$ 92.0				$ 102.0
Finance Lease, Liability	274.0				246.2
Senior Notes	13,253.0				12,103.0
Junior Subordinated Notes	2,000.0				1,000.0
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(141.5)				(95.5)
Long-Term Debt, Current Maturities	(19.7)				(1,281.2)
Long-term Debt, Excluding Current Maturities	15,477.5				13,355.7
Long-term Debt and Lease Obligation	$ 15,457.8				12,074.5
NiSource
Debt Instrument [Line Items]
Senior Notes				$ 900.0
NiSource \| 0.95% Notes Due 2025
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	0.95%
Senior Notes	$ 0.0				1,250.0
NiSource \| 3.49% Notes due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.49%
Senior Notes	$ 1,000.0				1,000.0
NiSource \| 6.78% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.78%
Senior Notes	$ 3.0				3.0
NiSource \| 2.95% Notes Due 2029
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.95%
Senior Notes	$ 750.0				750.0
NiSource \| 3.60% Notes Due 2030
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.60%
Senior Notes	$ 1,000.0				1,000.0
NiSource \| 1.70% Notes Due 2031
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	1.70%
Senior Notes	$ 750.0				750.0
NiSource \| 6.25% Notes Due 2040
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.25%
Senior Notes	$ 152.6				152.6
NiSource \| 5.95% Notes Due 2041
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.95%
Senior Notes	$ 347.4				347.4
NiSource \| 5.80% Notes Due 2042
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.80%
Senior Notes	$ 250.0				250.0
NiSource \| 5.25% Notes Due 2043
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.25%
Senior Notes	$ 500.0				500.0
NiSource \| 4.80% Notes due 2044
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.80%
Senior Notes	$ 750.0				750.0
NiSource \| 5.65% Notes Due 2045
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.65%
Senior Notes	$ 500.0				500.0
NiSource \| 4.375% Notes due 2047
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.375%
Senior Notes	$ 1,000.0				1,000.0
NiSource \| 3.95% Notes Due 2048
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.95%
Senior Notes	$ 750.0				750.0
NiSource \| Medium-Term Note
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	7.99%
Medium-Term Notes	$ 29.0				29.0
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Debt, Weighted Average Interest Rate	5.00%
Senior Notes	$ 350.0				350.0
NiSource \| 5.25% Notes Due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.25%
Senior Notes	$ 1,050.0				1,050.0
NiSource \| 5.40% Notes Due 2033
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.40%
Senior Notes	$ 450.0				450.0
NiSource \| 5.20% Notes Due 2029
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.20%						5.20%
Senior Notes	$ 600.0				600.0		$ 600.0
NiSource \| 5.35% Notes Due 2034
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.35%								5.35%
Senior Notes	$ 650.0				650.0				$ 650.0
NiSource \| 6.95% Notes due 2054
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Debt, Weighted Average Interest Rate	6.95%							6.95%
Junior Subordinated Notes	$ 500.0				500.0			$ 500.0
NiSource \| 6.375% Notes due 2054
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.375%					6.375%
Junior Subordinated Notes	$ 500.0				500.0	$ 500.0
NiSource \| 5.35% Notes Due 2035
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.35%
Senior Notes	$ 900.0				0.0
NiSource \| 5.85% Notes Due 2055
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.85%
Senior Notes	$ 1,500.0				0.0
NiSource \| Notes due 2056
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.75%		5.75%
Junior Subordinated Notes	$ 1,000.0		$ 1,000.0		0.0
Columbia Of Massachusetts \| Medium-Term Note
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		6.43%
Columbia Of Massachusetts \| Medium Term Note due 2025
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.43%
Medium-Term Notes	$ 0.0				10.0
Columbia Of Massachusetts \| Medium Term Note due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.26%
Medium-Term Notes	$ 5.0				5.0
NIPSCO \| Medium-Term Note
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	7.644%
Medium-Term Notes	$ 58.0				58.0
NIPSCO \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.48%
Finance Lease, Liability	$ 165.5				124.3
Columbia Of Ohio \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.11%
Finance Lease, Liability	$ 81.7				85.6
NiSource Corporate Services \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.35%
Finance Lease, Liability	$ 5.0				14.6
Columbia Of Virginia \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.62%
Finance Lease, Liability	$ 16.2				15.2
Columbia Of Kentucky \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.36%
Finance Lease, Liability	$ 0.1				0.1
Columbia Of Pennsylvania \| Finance Leases
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.43%
Finance Lease, Liability	$ 5.5				$ 6.4