Revenue Recognition (Revenue Recognition Under Previous Guidance) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Revenue Recognition Under Previous Guidance [Line Items]
Other Revenues	$ 33.6	$ 56.1
Total Operating Revenues	1,750.8	1,598.6
Previous Accounting Guidance
Revenue Recognition Under Previous Guidance [Line Items]
Gas Distribution	983.8	836.5
Gas Transportation	342.2	338.6
Electric	423.3	421.7
Other Revenues	1.5	1.8
Total Operating Revenues	$ 1,750.8	$ 1,598.6

[Line Items] for Revenue Recognition Under Previous Guidance [Table]

No definition available.

Name:	ni_RevenueRecognitionUnderPreviousGuidanceLineItems
Namespace Prefix:	ni_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_ElectricDomesticRegulatedRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of [natural] gas.

+ References

+ Details

Name:	us-gaap_GasDomesticRegulatedRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Midstream refers to those gas industry activities that fall between exploration and production (upstream) and refining and marketing (downstream). It may be applied to the gathering, processing, transmission and storage of natural gas; including the treating of natural gas to remove impurities, along with the sale of by-products such as residue gas, natural gas liquids and condensate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 23
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62246-109447

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_NaturalGasMidstreamRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after sales adjustments, returns, allowances, and discounts, of revenue classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(a),(d))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_OtherSalesRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_SalesRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_ProspectiveAdoptionOfNewAccountingPronouncementsAxis=us-gaap_PreviousAccountingGuidanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: