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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,790.3 us-gaap_RegulatoryAssets | $ 1,783.7 us-gaap_RegulatoryAssets
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 968.2 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 955.3 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 562.9 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember | 570.2 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 75.6 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember | 73.7 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 49.6 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember | 46.9 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember
Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 32.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 31.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.9 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember | 27.4 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 75.0 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 79.1 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,363.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember | 1,357.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 940.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 928.0 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 | 284.8 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember | 287.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 42.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember | 41.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember | 28.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0.6 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 0.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember | 27.4 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = lnt_EmissionAllowancesMember
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 42.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | 44.8 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 426.6 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember | 425.8 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_TaxRelatedMember | 27.3 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 | 278.1 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember | 282.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 33.1 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember | 32.3 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRetirementObligationCostsMember
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 23.2 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember | 18.9 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_DerivativeMember
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 31.5 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember | 30.7 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = lnt_CommodityCostRecoveryMember
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 33.0 us-gaap_RegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | $ 34.3 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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