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Regulatory Matters (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 821.2 us-gaap_RegulatoryLiabilities | $ 821.5 us-gaap_RegulatoryLiabilities
Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 421.7 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember | 418.9 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember
IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 243.0 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember | 265.4 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember
Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 64.3 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember | 52.7 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember
IPL's electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 19.4 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplSElectricTransmissionServiceCostsMember | 14.6 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplSElectricTransmissionServiceCostsMember
Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 15.4 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember | 7.5 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember
IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 11.3 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember | 21.6 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember
Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 46.1 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 40.8 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
IPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 583.5 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember | 614.9 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 279.1 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember | 277.7 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember
IPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 243.0 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember | 265.4 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember
IPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 0 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember | 9.3 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember
IPL [Member] | IPL's electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 19.4 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSElectricTransmissionServiceCostsMember | 14.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSElectricTransmissionServiceCostsMember
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 15.1 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember | 5.5 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember
IPL [Member] | IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 11.3 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember | 21.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember
IPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 15.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 20.8 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
WPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 237.7 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember | 206.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 142.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember | 141.2 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CostOfRemovalObligationsMember
WPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 64.3 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember | 43.4 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_EnergyConservationCostRecoveryMember
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 0.3 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember | 2.0 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = lnt_CommodityCostRecoveryMember
WPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 30.5 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | $ 20.0 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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