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Regulatory Matters (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 8 Months Ended 60 Months Ended 1 Months Ended 12 Months Ended 36 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Feb. 28, 2010
Dec. 31, 2014
Jan. 31, 2013
Jun. 30, 2012
Dec. 31, 2007
Dec. 31, 2011
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2016
Regulatory Matters [Line Items]                        
Minimum future commitments $ 2,360us-gaap_LongTermPurchaseCommitmentAmount                      
Increase in regulatory assets 439.8us-gaap_IncreaseDecreaseInOtherRegulatoryAssets (140.5)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets 178.1us-gaap_IncreaseDecreaseInOtherRegulatoryAssets                  
Increase (decrease) in regulatory liabilities 10.8us-gaap_IncreaseDecreaseInRegulatoryLiabilities (90.8)us-gaap_IncreaseDecreaseInRegulatoryLiabilities 16.4us-gaap_IncreaseDecreaseInRegulatoryLiabilities                  
Electric revenues 2,713.6us-gaap_ElectricDomesticRegulatedRevenue 2,689.0us-gaap_ElectricDomesticRegulatedRevenue 2,589.3us-gaap_ElectricDomesticRegulatedRevenue                  
Regulatory liabilities 821.2us-gaap_RegulatoryLiabilities 821.5us-gaap_RegulatoryLiabilities     821.2us-gaap_RegulatoryLiabilities              
Gas revenues 517.5us-gaap_GasDomesticRegulatedRevenue 464.8us-gaap_GasDomesticRegulatedRevenue 396.3us-gaap_GasDomesticRegulatedRevenue                  
IPL's electric transmission assets sale [Member]                        
Regulatory Matters [Line Items]                        
Regulatory liabilities 11.3us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		 21.6us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		     11.3us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		              
IPL [Member]                        
Regulatory Matters [Line Items]                        
Regulatory assets not earning a return 26us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
/ dei_LegalEntityAxis
= lnt_IplMember		       26us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
/ dei_LegalEntityAxis
= lnt_IplMember		              
Period for simple average of actual costs incurred during test year 2 years                      
Minimum future commitments 1,859us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lnt_IplMember		                      
Increase in regulatory assets 272.9us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_IplMember		 (71.4)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_IplMember		 129.0us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_IplMember		                  
Number of tax accounting method changes (number of changes) 2lnt_NumberOfTaxAccountingMethodChanges
/ dei_LegalEntityAxis
= lnt_IplMember		                      
Increase (decrease) in regulatory liabilities (18.9)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		 (82.3)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		 (12.1)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		                  
Electric revenues 1,493.3us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		 1,491.8us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		 1,371.1us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		                  
Regulatory liabilities 583.5us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		 614.9us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		     583.5us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember		              
Gas revenues 296.5us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		 273.9us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		 226.7us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_IplMember		                  
Billing credits to reduce retail electric customers' bills 72lnt_CustomerBillingCredits
/ dei_LegalEntityAxis
= lnt_IplMember		                      
IPL [Member] | IPL's electric transmission assets sale [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities (8.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		       41.0us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		              
Reduction of Iowa retail electric customers' rates       12lnt_ReductionInElectricCustomerRates
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		                
Regulatory liabilities 11.3us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		 21.6us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		     11.3us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplElectricTransmissionAssetsSaleMember		              
Regulatory liability amortization period 5 years                      
IPL [Member] | 2011 Test Year Retail Gas IA, Request [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount           11us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_IplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember		            
Period after commencement of rate case proceeding 12 months                      
Authorized increase (decrease) in interim rates, amount             9us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_IplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember		          
Authorized increase (decrease) in interim rates, percent             3.00%us-gaap_PublicUtilitiesInterimRateIncreaseDecreasePercentage
/ dei_LegalEntityAxis
= lnt_IplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember		          
Authorized increase (decrease) in final rates, percent           4.00%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
/ dei_LegalEntityAxis
= lnt_IplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember		            
WPL [Member]                        
Regulatory Matters [Line Items]                        
Regulatory assets not earning a return 10us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
/ dei_LegalEntityAxis
= lnt_WplMember		       10us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
/ dei_LegalEntityAxis
= lnt_WplMember		              
Minimum future commitments 499us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lnt_WplMember		                      
Increase in regulatory assets 166.9us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_WplMember		 (69.1)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_WplMember		 49.1us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_WplMember		                  
Expenditure collection period after new rate implementation 5 years                      
Increase (decrease) in regulatory liabilities 29.7us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		 (8.5)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		 28.5us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		                  
Electric revenues 1,220.3us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		 1,197.2us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		 1,218.2us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		                  
Regulatory liabilities 237.7us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		 206.6us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		     237.7us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		              
Gas revenues 221.0us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		 190.9us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		 169.6us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_WplMember		                  
WPL [Member] | 2013/2014 Test Period Retail Electric And Gas Final Order [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount   13us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenAndFourteenTestPeriodRetailElectricAndGasFinalOrderMember		                    
WPL [Member] | 2014 Test Year Retail Electric Fuel Related Request [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount 19us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		                      
Authorized increase (decrease) in final rates, percent 2.00%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		                      
Annual bandwidth for fuel-related costs 2.00%lnt_RateOfFuelCostsCapacity
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		                      
WPL [Member] | 2013 Test Year Retail Electric Fuel Related Request [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount   29us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenTestYearRetailElectricFuelRelatedRequestMember		                    
Authorized increase (decrease) in final rates, percent   3.00%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenTestYearRetailElectricFuelRelatedRequestMember		                    
Annual bandwidth for fuel-related costs   2.00%lnt_RateOfFuelCostsCapacity
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenTestYearRetailElectricFuelRelatedRequestMember		                    
WPL [Member] | 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount     4us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember		                  
Annual bandwidth for fuel-related costs     2.00%lnt_RateOfFuelCostsCapacity
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember		                  
Over-collection of fuel-related costs     17lnt_OverCollectionOfRetailFuelRelatedCosts
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember		                  
Over-collection of fuel-related costs outside of approved range     11us-gaap_CustomerRefundLiabilityCurrent
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember		                  
Authorized refund to customers, including interest   12lnt_PublicUtilitiesRefundToCustomersIncludingInterest
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember		                    
WPL [Member] | WPL's Edgewater Unit 5 Purchase [Member]                        
Regulatory Matters [Line Items]                        
Ownership interest % 25.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilityAxis
= lnt_WplSEdgewaterUnitFivePurchaseMember		       25.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilityAxis
= lnt_WplSEdgewaterUnitFivePurchaseMember		              
Alliant Energy and IPL [Member] | Tax accounting method changes [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities 75.0us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_TaxAccountingMethodChangeMember		                      
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member]                        
Regulatory Matters [Line Items]                        
Electric revenues 15.0us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_RevenueRequirementAdjustmentMember		 24.0us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_RevenueRequirementAdjustmentMember		                    
Alliant Energy and IPL [Member] | 2011 Test Year Retail Gas IA, Request [Member]                        
Regulatory Matters [Line Items]                        
Gas revenues     5.0us-gaap_GasDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember		                  
Alliant Energy and WPL [Member] | Deferral [Member]                        
Regulatory Matters [Line Items]                        
Regulatory liabilities 9.0us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_DeferralMember		       9.0us-gaap_RegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_DeferralMember		              
Alliant Energy and WPL [Member] | 2013/2014 Test Period Retail Electric And Gas Final Order [Member]                        
Regulatory Matters [Line Items]                        
Increase in regulatory assets     5.0us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenAndFourteenTestPeriodRetailElectricAndGasFinalOrderMember		                  
Credit to utility other operation and maintenance     5lnt_CreditToUtilityOtherOperationAndMaintenance
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandThirteenAndFourteenTestPeriodRetailElectricAndGasFinalOrderMember		                  
SO2 emission allowances [Member]                        
Regulatory Matters [Line Items]                        
Minimum future commitments 34us-gaap_LongTermPurchaseCommitmentAmount
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= lnt_So2EmissionAllowancesMember		 [1]                      
SO2 emission allowances [Member] | IPL [Member]                        
Regulatory Matters [Line Items]                        
Minimum future commitments 34us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= lnt_So2EmissionAllowancesMember		 [1]             34us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= lnt_So2EmissionAllowancesMember		        
SO2 emission allowances [Member] | Alliant Energy and IPL [Member]                        
Regulatory Matters [Line Items]                        
Minimum future commitments                 34us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= lnt_So2EmissionAllowancesMember		      
Increase in regulatory assets                 30.0us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= lnt_So2EmissionAllowancesMember		      
Scenario, Forecast [Member] | IPL [Member]                        
Regulatory Matters [Line Items]                        
Billing credits to reduce retail electric customers' bills                   10lnt_CustomerBillingCredits
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 25lnt_CustomerBillingCredits
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 105lnt_CustomerBillingCredits
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member]                        
Regulatory Matters [Line Items]                        
Electric revenues                     15.0us-gaap_ElectricDomesticRegulatedRevenue
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_RevenueRequirementAdjustmentMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Electric [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities (85.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_ElectricMember		 (79.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_ElectricMember		 (83.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_ElectricMember		                  
Electric [Member] | Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities                     (75.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_ElectricMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Gas [Member] | IPL's tax benefit riders [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities (12.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_GasMember		                      
Gas [Member] | IPL [Member] | 2011 Test Year Retail Gas IA, Request [Member] | IPL's tax benefit riders [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount 36us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ lnt_RetailRequestAxis
= lnt_TwoThousandElevenTestYearRetailGasRequestIaMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_GasMember		                      
Gas tax benefit rider term 3 years                      
Gas [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]                        
Regulatory Matters [Line Items]                        
Increase (decrease) in regulatory liabilities (12.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_GasMember		 (11.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_IplSTaxBenefitRiderMember
/ us-gaap_StatementBusinessSegmentsAxis
= lnt_GasMember		                    
2014 Test Year Retail Electric Fuel Related Request [Member] | Alliant Energy and WPL [Member]                        
Regulatory Matters [Line Items]                        
Over-collection of fuel-related costs 33lnt_OverCollectionOfRetailFuelRelatedCosts
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		                      
Over-collection of fuel-related costs outside of approved range 28us-gaap_CustomerRefundLiabilityCurrent
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		       28us-gaap_CustomerRefundLiabilityCurrent
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember		              
2015/2016 Test Period Retail Electric And Gas [Member] | Scenario, Forecast [Member] | WPL [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount                     (5)us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFifteenAndSixteenTestPeriodRetailElectricAndGasMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
2015 Test Year Retail Electric Fuel Related Request [Member] | Scenario, Forecast [Member] | WPL [Member]                        
Regulatory Matters [Line Items]                        
Authorized increase (decrease) in final rates, amount                     39us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFifteenTestYearRetailElectricFuelRelatedRequestMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Authorized increase (decrease) in final rates, percent                     4.00%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFifteenTestYearRetailElectricFuelRelatedRequestMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Annual bandwidth for fuel-related costs                     2.00%lnt_RateOfFuelCostsCapacity
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFifteenTestYearRetailElectricFuelRelatedRequestMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Fuel [Member] | Scenario, Forecast [Member] | WPL [Member]                        
Regulatory Matters [Line Items]                        
Retail electric fuel-related cots                     25us-gaap_DeferredFuelCost
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= us-gaap_FuelMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
Sales [Member] | Scenario, Forecast [Member] | WPL [Member]                        
Regulatory Matters [Line Items]                        
Retail electric fuel-related cots                     $ 14us-gaap_DeferredFuelCost
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= us-gaap_SalesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		  
[1] Refer to Note 2 for discussion of $34 million of charges recognized by Alliant Energy and IPL in 2011 for forward contracts to purchase SO2 emission allowances.