v2.4.0.8

Variable Interests and Variable Interest Entities (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended
--- | ---
Jun. 30, 2013 | Jun. 30, 2012
Schedule of Equity Method Investments [Line Items] |  |
Payments made to Pure Air | $ 6.4 | $ 10.7

X
- Definition No authoritative reference available. + References No definition available. + Details
Name: | ni_PaymentsPureAir
Namespace Prefix: | ni_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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