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Property, Plant and Equipment (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
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 $ 34.8us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions $ 30.8us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions $ 21.9us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions    
Construction work in progress 479.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress 677.9us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress      
Non-regulated Generation, net 240.1lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 [1] 249.4lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 [1]      
Corporate Services and other, net 269.4lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 [2] 229.6lnt_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 25.9us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_IplMember		 21.0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_IplMember		 8.4us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_IplMember		    
Construction work in progress 325.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ dei_LegalEntityAxis
= lnt_IplMember		 346.4us-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 8.9us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_WplMember		 9.8us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_WplMember		 13.5us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_WplMember		    
Construction work in progress 154.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ dei_LegalEntityAxis
= lnt_WplMember		 331.5us-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 137.0lnt_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 103.0lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1
/ dei_LegalEntityAxis
= lnt_SheboyganFallsEnergyFacilityMember		        
Whispering Willow - East Wind Project [Member]          
Property, Plant and Equipment [Line Items]          
Electric plant in service   7.0us-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 8us-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       8us-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 60.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_PublicUtilityAxis
= lnt_GeorgeNealUnit3Member		        
Allowance for funds used during construction 4.0us-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 272.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilityAxis
= lnt_ColumbiaUnits1and2Member		        
Allowance for funds used during construction 15.0us-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 3.0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilityAxis
= lnt_EdgewaterUnit5Member		        
Construction work in progress 90.0us-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) 650lnt_FossilFueledEnergyGeneratingUnitsCapacity
/ us-gaap_PublicUtilityAxis
= lnt_MarshalltownGeneratingStationMember		        
Marshalltown Generating Station [Member] | IPL [Member]          
Property, Plant and Equipment [Line Items]          
Allowance for funds used during construction 3.7us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_PublicUtilityAxis
= lnt_MarshalltownGeneratingStationMember		 0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_PublicUtilityAxis
= lnt_MarshalltownGeneratingStationMember		 0us-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 4.0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_PublicUtilityAxis
= lnt_MarshalltownGeneratingStationMember		        
Construction work in progress 188.0us-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 30lnt_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 154.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_JointlyOwnedUtilityPlantAxis
= lnt_OttumwaUnit1Member
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember		        
Allowance for funds used during construction 21.0us-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         130us-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         10us-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 1us-gaap_AccumulatedCapitalizedInterestCosts
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= lnt_CustomerBillingAndInformationSystemMember		        
Corporate Services and other, net $ 62.0lnt_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.