Long-Term Debt (Schedule of Consolidated Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Senior Notes	$ 9,003.0	$ 7,581.6
Medium-Term Notes	132.0	157.0
Capital Lease Obligations	195.0	201.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(86.9)	(70.5)
Long-term Debt, Excluding Current Maturities	$ 9,243.1	7,869.6
NiSource \| 4.45% Notes Due 2021
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2021
Debt, Weighted Average Interest Rate	4.45%
Senior Notes	$ 0.0	63.6
NiSource \| 2.65% Notes Due 2022
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2022
Debt, Weighted Average Interest Rate	2.65%
Senior Notes	$ 0.0	500.0
NiSource \| 3.85% Notes Due 2023
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2023
Debt, Weighted Average Interest Rate	3.85%
Senior Notes	$ 0.0	250.0
NiSource \| 3.65% Notes Due 2023
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jun. 30, 2023
Debt, Weighted Average Interest Rate	3.65%
Senior Notes	$ 0.0	350.0
NiSource \| 0.95% Notes Due 2025
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	0.95%
Senior Notes	$ 1,250.0	0.0
NiSource \| 5.89% Notes Due 2025
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2025
Debt, Weighted Average Interest Rate	5.89%
Senior Notes	$ 0.0	265.0
NiSource \| 3.49% Notes due 2027
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2027
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 Instrument, Maturity Date	Dec. 31, 2027
Debt, Weighted Average Interest Rate	6.78%
Senior Notes	$ 3.0	3.0
NiSource \| 2.95% Notes Due 2029
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 30, 2029
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	0.0
NiSource \| 1.70% Notes Due 2031
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	1.70%
Senior Notes	$ 750.0	0.0
NiSource \| 6.25% Notes Due 2040
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2040
Debt, Weighted Average Interest Rate	6.25%
Senior Notes	$ 152.6	250.0
NiSource \| 5.95% Notes Due 2041
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jun. 30, 2041
Debt, Weighted Average Interest Rate	5.95%
Senior Notes	$ 347.4	400.0
NiSource \| 5.80% Notes Due 2042
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2042
Debt, Weighted Average Interest Rate	5.80%
Senior Notes	$ 250.0	250.0
NiSource \| 5.25% Notes Due 2043
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2043
Debt, Weighted Average Interest Rate	5.25%
Senior Notes	$ 500.0	500.0
NiSource \| 4.80% Notes due 2044
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 29, 2044
Debt, Weighted Average Interest Rate	4.80%
Senior Notes	$ 750.0	750.0
NiSource \| 5.65% Notes Due 2045
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2045
Debt, Weighted Average Interest Rate	5.65%
Senior Notes	$ 500.0	500.0
NiSource \| 4.375% Notes due 2047
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2047
Debt, Weighted Average Interest Rate	4.38%
Senior Notes	$ 1,000.0	1,000.0
NiSource \| 3.95% Notes Due 2048
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 31, 2048
Debt, Weighted Average Interest Rate	3.95%
Senior Notes	$ 750.0	750.0
NiSource \| 7.99% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	7.99%
Medium-Term Notes	$ 49.0	$ 49.0
NiSource \| 7.99% Notes Due 2027 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2027
NiSource \| 7.99% Notes Due 2027 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Apr. 30, 2022
Columbia Of Massachusetts \| 6.30% Notes Due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.37%	630.00%
Medium-Term Notes	$ 15.0	$ 40.0
Columbia Of Massachusetts \| 6.30% Notes Due 2028 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2028
Columbia Of Massachusetts \| 6.30% Notes Due 2028 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2025
Columbia Of Massachusetts \| 5.49% notes Due 2043
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	0.00%
Capital Lease Obligations	$ 0.0	44.3
Columbia Of Massachusetts \| 5.49% notes Due 2043 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2043
Columbia Of Massachusetts \| 5.49% notes Due 2043 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2033
NIPSCO \| 7.61% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	7.61%
Medium-Term Notes	$ 68.0	68.0
NIPSCO \| 7.61% Notes Due 2027 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Aug. 31, 2027
NIPSCO \| 7.61% Notes Due 2027 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Aug. 31, 2022
NIPSCO \| 1.79% Notes Due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	1.79%
Capital Lease Obligations	$ 16.0	0.0
NIPSCO \| 5.85% Notes Due 2019 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Apr. 30, 2019
Columbia Of Ohio \| 6.16% Notes Due 2044
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.16%
Capital Lease Obligations	$ 91.2	94.8
Columbia Of Ohio \| 6.16% Notes Due 2044 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 31, 2044
Columbia Of Ohio \| 6.16% Notes Due 2044 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Oct. 31, 2021
NiSource Corporate Services \| 3.47% Notes Due 2023
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.19%
Capital Lease Obligations	$ 49.4	22.3
NiSource Corporate Services \| 3.47% Notes Due 2023 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2023
NiSource Corporate Services \| 3.47% Notes Due 2023 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jan. 31, 2020
Columbia Of Virginia \| 6.31% Notes Due 2039
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.30%
Capital Lease Obligations	$ 18.4	19.1
Columbia Of Virginia \| 6.31% Notes Due 2039 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2039
Columbia Of Virginia \| 6.31% Notes Due 2039 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jul. 31, 2029
Columbia Of Kentucky \| 3.79% Notes Due 2027
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2027
Debt, Weighted Average Interest Rate	3.79%
Capital Lease Obligations	$ 0.3	0.3
Columbia Of Pennsylvania \| 5.67% Notes Due 2035
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.65%
Capital Lease Obligations	$ 19.7	$ 20.7
Columbia Of Pennsylvania \| 5.67% Notes Due 2035 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2035
Columbia Of Pennsylvania \| 5.67% Notes Due 2035 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Aug. 31, 2027

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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