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Quarterly Financial Data (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Quarterly Financial Data [Line Items]      
Gain (Loss) on Disposition of Discontinued Operations - net of taxes $ 0us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax $ 34.9us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax $ 0us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
Service Plan And Leasing Business Lines of Retail Services Business [Member]      
Quarterly Financial Data [Line Items]      
Gain (Loss) on Disposition of Discontinued Operations - net of taxes   $ 36.4us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= ni_ServicePlanAndLeasingBusinessLinesofRetailServicesBusinessMember