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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
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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 | $ 2,360 us-gaap_LongTermPurchaseCommitmentAmount |  |  |  |  |  |  |  |  |  |  |  |
Increase in regulatory assets | 439.8 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets |  | (140.5)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets | 178.1 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets |  |  |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities | 10.8 us-gaap_IncreaseDecreaseInRegulatoryLiabilities |  | (90.8)us-gaap_IncreaseDecreaseInRegulatoryLiabilities | 16.4 us-gaap_IncreaseDecreaseInRegulatoryLiabilities |  |  |  |  |  |  |  |  |
Electric revenues | 2,713.6 us-gaap_ElectricDomesticRegulatedRevenue |  | 2,689.0 us-gaap_ElectricDomesticRegulatedRevenue | 2,589.3 us-gaap_ElectricDomesticRegulatedRevenue |  |  |  |  |  |  |  |  |
Regulatory liabilities | 821.2 us-gaap_RegulatoryLiabilities |  | 821.5 us-gaap_RegulatoryLiabilities |  |  | 821.2 us-gaap_RegulatoryLiabilities |  |  |  |  |  |  |
Gas revenues | 517.5 us-gaap_GasDomesticRegulatedRevenue |  | 464.8 us-gaap_GasDomesticRegulatedRevenue | 396.3 us-gaap_GasDomesticRegulatedRevenue |  |  |  |  |  |  |  |  |
IPL's electric transmission assets sale [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities | 11.3 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  | 21.6 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  |  | 11.3 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  |  |  |  |  |  |
IPL [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets not earning a return | 26 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  | 26 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |
Period for simple average of actual costs incurred during test year | 2 years |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments | 1,859 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |  |  |  |  |  |
Increase in regulatory assets | 272.9 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember |  | (71.4)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember | 129.0 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |  |  |
Number of tax accounting method changes (number of changes) | 2 lnt_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 | 1,493.3 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember |  | 1,491.8 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember | 1,371.1 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |  |  |
Regulatory liabilities | 583.5 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember |  | 614.9 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember |  |  | 583.5 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |
Gas revenues | 296.5 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember |  | 273.9 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember | 226.7 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |  |  |  |  |
Billing credits to reduce retail electric customers' bills | 72 lnt_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 |  |  |  |  | 41.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  |  |  |  |  |  |
Reduction of Iowa retail electric customers' rates |  |  |  |  | 12 lnt_ReductionInElectricCustomerRates / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  |  |  |  |  |  |  |
Regulatory liabilities | 11.3 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  | 21.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplElectricTransmissionAssetsSaleMember |  |  | 11.3 us-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 |  |  |  |  |  |  | 11 us-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 |  |  |  |  |  |  |  | 9 us-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 | 10 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  | 10 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |
Minimum future commitments | 499 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |  |  |  |  |  |
Increase in regulatory assets | 166.9 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember |  | (69.1)us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember | 49.1 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |  |  |
Expenditure collection period after new rate implementation | 5 years |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities | 29.7 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |  | (8.5)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember | 28.5 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |  |  |
Electric revenues | 1,220.3 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_WplMember |  | 1,197.2 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_WplMember | 1,218.2 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |  |  |
Regulatory liabilities | 237.7 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |  | 206.6 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |  |  | 237.7 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |  |  |  |  |  |  |
Gas revenues | 221.0 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_WplMember |  | 190.9 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_WplMember | 169.6 us-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 |  |  | 13 us-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 | 19 us-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 |  |  | 29 us-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 |  |  |  | 4 us-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 |  |  |  | 17 lnt_OverCollectionOfRetailFuelRelatedCosts / dei_LegalEntityAxis = lnt_WplMember / lnt_RetailRequestAxis = lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember |  |  |  |  |  |  |  |  |
Over-collection of fuel-related costs outside of approved range |  |  |  | 11 us-gaap_CustomerRefundLiabilityCurrent / dei_LegalEntityAxis = lnt_WplMember / lnt_RetailRequestAxis = lnt_TwoThousandTwelveTestYearRetailElectricFuelRelatedRequestFinalOrderMember |  |  |  |  |  |  |  |  |
Authorized refund to customers, including interest |  |  | 12 lnt_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 | 75.0 us-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 | 15.0 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_RevenueRequirementAdjustmentMember |  | 24.0 us-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 |  |  |  | 5.0 us-gaap_GasDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / lnt_RetailRequestAxis = lnt_TwoThousandElevenTestYearRetailGasRequestIaMember |  |  |  |  |  |  |  |  |
Alliant Energy and WPL [Member] | Deferral [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities | 9.0 us-gaap_RegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_RegulatoryLiabilityAxis = lnt_DeferralMember |  |  |  |  | 9.0 us-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 |  |  |  | 5.0 us-gaap_IncreaseDecreaseInOtherRegulatoryAssets / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / lnt_RetailRequestAxis = lnt_TwoThousandThirteenAndFourteenTestPeriodRetailElectricAndGasFinalOrderMember |  |  |  |  |  |  |  |  |
Credit to utility other operation and maintenance |  |  |  | 5 lnt_CreditToUtilityOtherOperationAndMaintenance / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / lnt_RetailRequestAxis = lnt_TwoThousandThirteenAndFourteenTestPeriodRetailElectricAndGasFinalOrderMember |  |  |  |  |  |  |  |  |
SO2 emission allowances [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments | 34 us-gaap_LongTermPurchaseCommitmentAmount / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_So2EmissionAllowancesMember | [1] |  |  |  |  |  |  |  |  |  |  |
SO2 emission allowances [Member] | IPL [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments | 34 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_So2EmissionAllowancesMember | [1] |  |  |  |  |  |  | 34 us-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 |  |  |  |  |  |  |  |  |  | 34 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_So2EmissionAllowancesMember |  |  |
Increase in regulatory assets |  |  |  |  |  |  |  |  |  | 30.0 us-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 |  |  |  |  |  |  |  |  |  |  | 10 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember | 25 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember | 105 lnt_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 |  |  |  |  |  |  |  |  |  |  |  | 15.0 us-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 | 36 us-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 | 33 lnt_OverCollectionOfRetailFuelRelatedCosts / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember |  |  |  |  |  |  |  |  |  |  |  |
Over-collection of fuel-related costs outside of approved range | 28 us-gaap_CustomerRefundLiabilityCurrent / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember |  |  |  |  | 28 us-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 |  |  |  |  |  |  |  |  |  |  |  | 39 us-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 |  |  |  |  |  |  |  |  |  |  |  | 25 us-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 |  |  |  |  |  |  |  |  |  |  |  | $ 14 us-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.

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