Long-Term Debt (Schedule of Consolidated Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Senior Notes	$ 6,831.6	$ 7,516.9
Medium-Term Notes	157.0	157.0
Capital Lease Obligations	194.3	153.1
Long-term Debt, Excluding Current Maturities	7,155.4	7,796.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (68.5)	(71.5)
4.375% Notes due 2047
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2027
3.68% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Oct. 31, 2022
NiSource Finance \| 6.40% Notes Due 2018
Debt Instrument [Line Items]
Senior Notes		275.1
NiSource Finance \| 6.80% Notes Due 2019
Debt Instrument [Line Items]
Senior Notes		255.1
NiSource Finance \| 5.45% Notes due 2020
Debt Instrument [Line Items]
Senior Notes		325.1
NiSource Finance \| 4.45% Notes Due 2021
Debt Instrument [Line Items]
Senior Notes		63.6
NiSource Finance \| 6.13% Notes Due 2022
Debt Instrument [Line Items]
Senior Notes		180.0
NiSource Finance \| 2.65% Notes Due 2022
Debt Instrument [Line Items]
Senior Notes		500.0
NiSource Finance \| 3.85% Notes Due 2023
Debt Instrument [Line Items]
Senior Notes		250.0
NiSource Finance \| 3.65% Notes Due 2023
Debt Instrument [Line Items]
Senior Notes		0.0
NiSource Finance \| 5.89% Notes Due 2025
Debt Instrument [Line Items]
Senior Notes		265.0
NiSource Finance \| 3.49% Notes due 2027
Debt Instrument [Line Items]
Senior Notes		1,000.0
NiSource Finance \| 6.25% Notes Due 2040
Debt Instrument [Line Items]
Senior Notes		250.0
NiSource Finance \| 5.95% Notes Due 2041
Debt Instrument [Line Items]
Senior Notes		400.0
NiSource Finance \| 5.80% Notes Due 2042
Debt Instrument [Line Items]
Senior Notes		250.0
NiSource Finance \| 5.25% Notes Due 2043
Debt Instrument [Line Items]
Senior Notes		500.0
NiSource Finance \| 4.80% Notes due 2044
Debt Instrument [Line Items]
Senior Notes		750.0
NiSource Finance \| 5.65% Notes Due 2045
Debt Instrument [Line Items]
Senior Notes		500.0
Nisource Capital Markets Inc \| 6.78% Notes Due 2027
Debt Instrument [Line Items]
Senior Notes		3.0
Nisource Capital Markets Inc \| 4.375% Notes due 2047
Debt Instrument [Line Items]
Senior Notes		1,000.0
Nisource Capital Markets Inc \| 3.95% Notes Due 2048
Debt Instrument [Line Items]
Senior Notes		750.0
NiSource \| 6.40% Notes Due 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 31, 2018
Debt, Weighted Average Interest Rate	6.40%
Senior Notes	$ 0.0
NiSource \| 6.80% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jan. 31, 2019
Debt, Weighted Average Interest Rate	6.80%
Senior Notes	$ 0.0
NiSource \| 5.45% Notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 30, 2020
Debt, Weighted Average Interest Rate	5.45%
Senior Notes	$ 0.0
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	$ 63.6
NiSource \| 6.13% Notes Due 2022
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 31, 2022
Debt, Weighted Average Interest Rate	6.13%
Senior Notes	$ 0.0
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	$ 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	$ 250.0
NiSource \| 3.65% Notes Due 2023
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.65%
Senior Notes	$ 350.0
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	$ 265.0
NiSource \| 3.49% Notes due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.49%
Senior Notes	$ 1,000.0
NiSource \| 6.78% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.78%
Senior Notes	$ 3.0
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	$ 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	$ 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
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
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
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
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
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
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
NiSource \| 3.95% Notes Due 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	May 31, 2018
Columbia Of Massachusetts \| 6.30% Notes Due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.30%
Medium-Term Notes	$ 40.0	40.0
Columbia Of Massachusetts \| 6.30% Notes Due 2028 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2025
Columbia Of Massachusetts \| 6.30% Notes Due 2028 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Feb. 28, 2028
Columbia Of Massachusetts \| 5.48% notes Due 2043
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.48%
Capital Lease Obligations	$ 45.7	22.8
Columbia Of Massachusetts \| 5.48% notes Due 2043 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Nov. 30, 2043
Columbia Of Massachusetts \| 5.48% 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 \| 3.95% Notes Due 2018
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.95%
Capital Lease Obligations	$ 0.0	3.8
NIPSCO \| 5.85% Notes Due 2019
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.85%
Long-term Pollution Control Bond	$ 41.0	41.0
NIPSCO \| 5.85% Notes Due 2019 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Apr. 30, 2019
NIPSCO \| 5.85% Notes Due 2019 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jul. 31, 2017
Columbia Of Ohio \| 6.33% Notes Due 2038
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.33%
Capital Lease Obligations	$ 91.5	88.5
Columbia Of Ohio \| 6.33% Notes Due 2038 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jun. 30, 2038
Columbia Of Ohio \| 6.33% Notes Due 2038 \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Oct. 31, 2021
NiSource Corporate Services \| 3.68% Notes Due 2019
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.68%
Capital Lease Obligations	$ 11.6	1.4
Columbia Of Virginia \| 7.12% Notes Due 2037
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	7.12%
Capital Lease Obligations	$ 15.2	5.2
Columbia Of Virginia \| 7.12% Notes Due 2037 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 31, 2037
Columbia Of Virginia \| 7.12% Notes Due 2037 \| 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	0.4
Columbia Of Pennsylvania \| 5.42% Notes Due 2036
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.42%
Capital Lease Obligations	$ 30.0	$ 31.0
Columbia Of Pennsylvania \| 5.42% Notes Due 2036 \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jun. 30, 2036
Columbia Of Pennsylvania \| 5.42% Notes Due 2036 \| 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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