v2.4.0.8

Business Segment Information (Narrative) (Details) | 6 Months Ended
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Jun. 30, 2013
Segment Reporting [Abstract] |
Primary business segments | 3
Number of counties in which electric service provided by Electric Operations | 20

X
- Definition Primary Business Segments + References No definition available. + Details
Name: | ni_PrimaryBusinessSegments
Namespace Prefix: | ni_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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
- Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration