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Accumulated Other Comprehensive Loss (Narrative) (Details) (Millennium Pipeline Company, L.L.C [Member], USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Millennium Pipeline Company, L.L.C [Member]
    
Accumulated Other Comprehensive Loss [Line Items]    
Other Comprehensive Income Unrecognized Gain Loss On Derivatives Arising During Period Net Of Tax $ 14.2ni_OtherComprehensiveIncomeUnrecognizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
/ dei_LegalEntityAxis
= ni_MillenniumMember		 $ 16.6ni_OtherComprehensiveIncomeUnrecognizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
/ dei_LegalEntityAxis
= ni_MillenniumMember
Debt Instrument, Maturity Date Jun. 01, 2025