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Discontinued Operations and Assets and Liabilities Held For Sale (Components Of Assets And Liabilities Held For Sale In Consolidated Balance Sheets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |
Current assets | $ 1.1 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent
Property, plant and equipment,net | 11.0 us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
Other assets | 7.0 us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherAssets
Total assets held for sale | 19.1 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation
Current liabilities | 1.0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent
Other liabilities | 7.1 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent
Total liabilities held for sale | 8.1 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation
Net assets held for sale | $ 11.0 lnt_DisposalGroupNetAssetsHeldForSale

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