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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,783.7 us-gaap_RegulatoryAssets | $ 1,413.2 us-gaap_RegulatoryAssets
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 955.3 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 829.7 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 570.2 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember | 355.3 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 73.7 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember | 65.7 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 46.9 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember | 21.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember
Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 31.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 2.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.4 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember | 30.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember
Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 16.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember | 17.9 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 16.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember | 25.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 47.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 66.5 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,357.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember | 1,113.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 928.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 798.6 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 287.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember | 174.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 41.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember | 36.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 28.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember | 5.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 0.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember | 30.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember
IPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 10.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember | 12.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 11.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember | 21.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 22.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 34.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 425.8 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember | 299.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 31.1 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 282.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember | 181.1 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_PensionAndOtherPostretirementBenefitsMember
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 32.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember | 29.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_AssetRetirementObligationsMember
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 18.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember | 15.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_DerivativesMember
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 1.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
WPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 5.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember | 5.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_DebtRedemptionCostsMember
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember | 4.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_EnvironmentalRelatedMember
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 24.6 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | $ 32.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets 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 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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