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Business Segment Information (Narrative) (Details) | 3 Months Ended
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Mar. 31, 2017
Segment Reporting [Abstract] |
Primary business segments | 2
Number of counties in which electric service provided by Electric Operations | 20

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- Definition Service Area By County + References No definition available. + Details
Name: | ni_ServiceAreaByCounty
Namespace Prefix: | ni_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration