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Jointly-Owned Electric Utility Plant (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Jointly Owned Electric Utility Plant [Line Items] |
Electric Plant | $ 1,488.1 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService
Accumulated Provision for Depreciation | 486.8 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation
Construction Work in Progress | 22.4 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress
Cost Of Removal Obligations Included In Regulatory Liabilities | 45.3 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities
IPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Electric Plant | 842.6 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / dei_LegalEntityAxis = lnt_IplMember
Accumulated Provision for Depreciation | 264.6 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / dei_LegalEntityAxis = lnt_IplMember
Construction Work in Progress | 3.2 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / dei_LegalEntityAxis = lnt_IplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 32.6 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Electric Plant | 645.5 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / dei_LegalEntityAxis = lnt_WplMember
Accumulated Provision for Depreciation | 222.2 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / dei_LegalEntityAxis = lnt_WplMember
Construction Work in Progress | 19.2 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / dei_LegalEntityAxis = lnt_WplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 12.7 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember
Ottumwa Unit 1 [Member] | IPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 48.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Electric Plant | 488.7 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Accumulated Provision for Depreciation | 131.6 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Construction Work in Progress | 1.4 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 11.0 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
George Neal Unit 4 [Member] | IPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 25.70%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit4Member / dei_LegalEntityAxis = lnt_IplMember
Electric Plant | 183.3 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit4Member / dei_LegalEntityAxis = lnt_IplMember
Accumulated Provision for Depreciation | 71.3 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit4Member / dei_LegalEntityAxis = lnt_IplMember
Construction Work in Progress | 0.3 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit4Member / dei_LegalEntityAxis = lnt_IplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 12.3 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit4Member / dei_LegalEntityAxis = lnt_IplMember
George Neal Unit 3 [Member] | IPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 28.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit3Member / dei_LegalEntityAxis = lnt_IplMember
Electric Plant | 135.2 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit3Member / dei_LegalEntityAxis = lnt_IplMember
Accumulated Provision for Depreciation | 41.3 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit3Member / dei_LegalEntityAxis = lnt_IplMember
Construction Work in Progress | 1.4 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit3Member / dei_LegalEntityAxis = lnt_IplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 6.0 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_GeorgeNealUnit3Member / dei_LegalEntityAxis = lnt_IplMember
Louisa Unit 1 [Member] | IPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 4.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_LouisaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Electric Plant | 35.4 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_LouisaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Accumulated Provision for Depreciation | 20.4 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_LouisaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Construction Work in Progress | 0.1 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_LouisaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 3.3 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_LouisaUnit1Member / dei_LegalEntityAxis = lnt_IplMember
Columbia Units 1-2 [Member] | WPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 46.20%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_ColumbiaUnits1And2Member / dei_LegalEntityAxis = lnt_WplMember
Electric Plant | 549.0 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_ColumbiaUnits1And2Member / dei_LegalEntityAxis = lnt_WplMember
Accumulated Provision for Depreciation | 166.8 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_ColumbiaUnits1And2Member / dei_LegalEntityAxis = lnt_WplMember
Construction Work in Progress | 18.9 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_ColumbiaUnits1And2Member / dei_LegalEntityAxis = lnt_WplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | 10.2 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_ColumbiaUnits1And2Member / dei_LegalEntityAxis = lnt_WplMember
Edgewater Unit 4 [Member] | WPL [Member] |
Jointly Owned Electric Utility Plant [Line Items] |
Ownership Interest % | 68.20%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_EdgewaterUnit4Member / dei_LegalEntityAxis = lnt_WplMember
Electric Plant | 96.5 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_EdgewaterUnit4Member / dei_LegalEntityAxis = lnt_WplMember
Accumulated Provision for Depreciation | 55.4 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_EdgewaterUnit4Member / dei_LegalEntityAxis = lnt_WplMember
Construction Work in Progress | 0.3 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_EdgewaterUnit4Member / dei_LegalEntityAxis = lnt_WplMember
Cost Of Removal Obligations Included In Regulatory Liabilities | $ 2.5 lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_EdgewaterUnit4Member / dei_LegalEntityAxis = lnt_WplMember

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- Definition Jointly Owned Utility Cost Of Removal Obligations Included In Regulatory Liabilities + References No definition available. + Details
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- Definition The period end amount owned by the reporting utility of a jointly owned utility plant in service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of construction work in progress at a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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