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v2.4.1.9

Income Taxes (Schedule Of Effective Income Tax Rates) (Details) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Effective Tax Rate [Line Items] |  |  |
Statutory federal income tax rate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefits | 5.40%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes | 5.70%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes | 5.70%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Effect of rate-making on property-related differences | (7.50%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences | (6.00%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences | (5.00%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences
Production tax credits | (7.10%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities | (7.10%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities | (5.80%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities
Adjustment of prior period taxes | (1.30%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes | (1.30%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes
Other items, net | (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments | (1.80%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments | (1.40%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Overall income tax rate | 10.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 12.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 20.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations
IPL [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
Statutory federal income tax rate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_IplMember | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_IplMember | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_IplMember
State income taxes, net of federal benefits | 5.00%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember | 5.40%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember | 5.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember
Effect of rate-making on property-related differences | (21.90%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_IplMember | (15.90%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_IplMember | (14.20%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_IplMember
Production tax credits | (9.80%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_IplMember | (9.50%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_IplMember | (9.60%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_IplMember
Adjustment of prior period taxes | (3.50%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember | (3.60%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember | 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_IplMember
Other items, net | (1.40%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_IplMember | (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_IplMember | (1.60%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_IplMember
Overall income tax rate | (36.20%)us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_IplMember | (24.90%)us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_IplMember | (15.20%)us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
Statutory federal income tax rate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_WplMember | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_WplMember | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate / dei_LegalEntityAxis = lnt_WplMember
State income taxes, net of federal benefits | 5.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember | 6.00%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember | 5.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember
Effect of rate-making on property-related differences | (0.70%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_WplMember | (0.80%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_WplMember | (1.10%)lnt_EffectiveIncomeTaxRateReconciliationEffectOfRateMakingOnPropertyRelatedDifferences / dei_LegalEntityAxis = lnt_WplMember
Production tax credits | (6.50%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_WplMember | (6.30%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_WplMember | (4.70%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsQualifiedProductionActivities / dei_LegalEntityAxis = lnt_WplMember
Adjustment of prior period taxes | (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember | (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember | (0.30%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes / dei_LegalEntityAxis = lnt_WplMember
Other items, net | (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_WplMember | (0.90%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_WplMember | (0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments / dei_LegalEntityAxis = lnt_WplMember
Overall income tax rate | 32.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_WplMember | 32.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_WplMember | 36.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations / dei_LegalEntityAxis = lnt_WplMember
IPL's tax benefit riders [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
IPL’s tax benefit riders | (12.90%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember | (12.10%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember | (11.20%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember
IPL's tax benefit riders [Member] | IPL [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
IPL’s tax benefit riders | (39.60%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember | (34.80%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember | (37.00%)lnt_EffectiveIncomeTaxRateReconciliationTaxBenefitRider / us-gaap_IncomeTaxAuthorityAxis = lnt_TaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember
State Apportionment [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
State apportionment change due to announced sale of RMT | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember | 3.50%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember
State Apportionment [Member] | IPL [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
State apportionment change due to announced sale of RMT | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_IplMember | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_IplMember | 6.20%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_IplMember
State Apportionment [Member] | WPL [Member] |  |  |
Effective Tax Rate [Line Items] |  |  |
State apportionment change due to announced sale of RMT | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_WplMember | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_WplMember | 2.70%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_WplMember

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