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Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
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Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | $ (13.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue | $ 17.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue | $ 23.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue | $ 4.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Excluding Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative liabilities | (17.2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | (7.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  |
Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | 4.1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | 24.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  |
IPL [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | (5.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 19.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 7.8 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 14.6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Excluding Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative liabilities | (8.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | (3.2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  |
IPL [Member] | Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | 3.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 22.6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  |
WPL [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | (8.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | (1.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | 16.1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | (10.2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Excluding Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative liabilities | (8.7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | (3.8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  |
WPL [Member] | Financial Transmission Rights [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, net derivative assets | $ 0.6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | $ 2.3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  |

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