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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | $ 31.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts | $ 31.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts | $ 24.8 lnt_ProductionTaxCreditsNetOfStateTaxImpacts
IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember | 14.4 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember | 12.5 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 17.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember | 16.6 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember | 12.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 13.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_BentTreePhaseIWindProjectMember / dei_LegalEntityAxis = lnt_WplMember | 12.5 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_BentTreePhaseIWindProjectMember / dei_LegalEntityAxis = lnt_WplMember | 8.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_BentTreePhaseIWindProjectMember / dei_LegalEntityAxis = lnt_WplMember
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_WhisperingWillowEastWindProjectMember / dei_LegalEntityAxis = lnt_IplMember | 14.4 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_WhisperingWillowEastWindProjectMember / dei_LegalEntityAxis = lnt_IplMember | 12.5 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis = lnt_WhisperingWillowEastWindProjectMember / dei_LegalEntityAxis = lnt_IplMember
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Wind EGU capacity (in megawatts) | 201 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_BentTreePhaseIWindProjectMember |  |
Cedar Ridge Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Wind EGU capacity (in megawatts) | 68 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember |  |
Production tax credits, net of state tax impacts | $ 4.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember | $ 4.1 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember | $ 4.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Wind EGU capacity (in megawatts) | 200 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_WhisperingWillowEastWindProjectMember |  |

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