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Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | $ 34.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions |  | $ 30.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions |  | $ 21.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions |  |
Construction work in progress | 479.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress |  | 677.9 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress |  |  |  |
Non-regulated Generation, net | 240.1 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 | [1] | 249.4 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 | [1] |  |  |
Corporate Services and other, net | 269.4 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 | [2] | 229.6 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 | [2] |  |  |
Minimum [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life | 5 years |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life | 30 years |  |  |  |  |  |
IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | 25.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember |  | 21.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember |  | 8.4 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember |  |
Construction work in progress | 325.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_IplMember |  | 346.4 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_IplMember |  |  |  |
Wholesale power supply agreement termination notice term | 5 years |  |  |  |  |  |
Wholesale power supply agreement term | 10 years |  |  |  |  |  |
WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | 8.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember |  | 9.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember |  | 13.5 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember |  |
Construction work in progress | 154.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_WplMember |  | 331.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_WplMember |  |  |  |
Franklin County Wind Project [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life | 30 years |  |  |  |  |  |
Non-regulated Generation, net | 137.0 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 / dei_LegalEntityAxis = lnt_FranklinCountyWindProjectMember |  |  |  |  |  |
Sheboygan Falls Energy Facility [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life | 35 years |  |  |  |  |  |
Non-regulated Generation, net | 103.0 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 / dei_LegalEntityAxis = lnt_SheboyganFallsEnergyFacilityMember |  |  |  |  |  |
Whispering Willow - East Wind Project [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric plant in service |  |  | 7.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = lnt_WhisperingWillowEastWindProjectMember |  |  |  |
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Disallowance of project costs | 8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAmountOfIndirectDisallowanceOfCostsOfRecentlyCompletedPlants / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_WhisperingWillowEastWindProjectMember |  |  |  |  |  |
Whispering Willow - East Wind Project [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Asset impairment charges |  |  |  |  |  | 8 us-gaap_AssetImpairmentCharges / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_WhisperingWillowEastWindProjectMember |
George Neal Unit 3 [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Ownership interest % | 28.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_GeorgeNealUnit3Member |  |  |  |  |  |
George Neal Unit 3 [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric plant in service | 60.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_GeorgeNealUnit3Member |  |  |  |  |  |
Allowance for funds used during construction | 4.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_GeorgeNealUnit3Member |  |  |  |  |  |
Columbia Units 1 and 2 [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Ownership interest % | 46.20%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_ColumbiaUnits1and2Member |  |  |  |  |  |
Columbia Units 1 and 2 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric plant in service | 272.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_ColumbiaUnits1and2Member |  |  |  |  |  |
Allowance for funds used during construction | 15.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_ColumbiaUnits1and2Member |  |  |  |  |  |
IPL (FERC formula) [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Authorized return on common equity | 10.97%lnt_AuthorizedReturnOnCommonEquity / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaOtherProjectsMember |  |  |  |  |  |
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | 3.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_EdgewaterUnit5Member |  |  |  |  |  |
Construction work in progress | 90.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_EdgewaterUnit5Member |  |  |  |  |  |
Marshalltown Generating Station [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | 650 lnt_FossilFueledEnergyGeneratingUnitsCapacity / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |  |  |  |  |
Marshalltown Generating Station [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | 3.7 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  | 0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  | 0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction | 4.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |  |  |  |  |
Construction work in progress | 188.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |  |  |  |  |
Minnesota [Member] | Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Project costs allocated by jurisdiction | 30 lnt_ProjectCostsAllocatedByJurisdiction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_WhisperingWillowEastWindProjectMember / us-gaap_StatementGeographicalAxis = stpr_MN |  |  |  |  |  |
Ottumwa Unit 1 [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Ownership interest % | 48.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |
Ottumwa Unit 1 [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric plant in service | 154.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember |  |  |  |  |  |
Allowance for funds used during construction | 21.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_JointlyOwnedUtilityPlantAxis = lnt_OttumwaUnit1Member / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember |  |  |  |  |  |
Electric - distribution [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Expected proceeds from assets sales |  |  |  |  |  |  | 130 us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricDistributionMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Gas - distribution [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Expected proceeds from assets sales |  |  |  |  |  |  | 10 us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasDistributionEquipmentMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Customer Billing And Information System [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Capitalized interest | 1 us-gaap_AccumulatedCapitalizedInterestCosts / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CustomerBillingAndInformationSystemMember |  |  |  |  |  |
Corporate Services and other, net | $ 62.0 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CustomerBillingAndInformationSystemMember |  |  |  |  |  |
[1] | Less accumulated depreciation of $49.5 million and $40.0 million for Alliant Energy as of December 31, 2014 and 2013, respectively.
[2] | Less accumulated depreciation of $229.1 million and $214.2 million for Alliant Energy as of December 31, 2014 and 2013, respectively.

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