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v2.4.1.9

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Current deferred tax assets | $ (150.1)us-gaap_DeferredTaxAssetsNetCurrent | $ (136.7)us-gaap_DeferredTaxAssetsNetCurrent
Non-current deferred tax liabilities | 2,321.1 us-gaap_DeferredTaxLiabilitiesNoncurrent | 2,112.7 us-gaap_DeferredTaxLiabilitiesNoncurrent
Deferred tax liabilities, net of deferred tax assets | 2,171.0 us-gaap_DeferredTaxLiabilities | 1,976.0 us-gaap_DeferredTaxLiabilities
IPL [Member] |  |
Deferred tax assets, gross | (376.4)us-gaap_DeferredTaxAssetsGross / dei_LegalEntityAxis = lnt_IplMember | (336.0)us-gaap_DeferredTaxAssetsGross / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 1,612.9 us-gaap_DeferredIncomeTaxLiabilities / dei_LegalEntityAxis = lnt_IplMember | 1,441.3 us-gaap_DeferredIncomeTaxLiabilities / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | 1,236.5 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / dei_LegalEntityAxis = lnt_IplMember | 1,105.3 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / dei_LegalEntityAxis = lnt_IplMember
Current deferred tax assets | (104.9)us-gaap_DeferredTaxAssetsNetCurrent / dei_LegalEntityAxis = lnt_IplMember | (87.7)us-gaap_DeferredTaxAssetsNetCurrent / dei_LegalEntityAxis = lnt_IplMember
Non-current deferred tax liabilities | 1,341.4 us-gaap_DeferredTaxLiabilitiesNoncurrent / dei_LegalEntityAxis = lnt_IplMember | 1,193.0 us-gaap_DeferredTaxLiabilitiesNoncurrent / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, net of deferred tax assets | 1,236.5 us-gaap_DeferredTaxLiabilities / dei_LegalEntityAxis = lnt_IplMember | 1,105.3 us-gaap_DeferredTaxLiabilities / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Deferred tax assets, gross | (244.4)us-gaap_DeferredTaxAssetsGross / dei_LegalEntityAxis = lnt_WplMember | (201.6)us-gaap_DeferredTaxAssetsGross / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 1,176.9 us-gaap_DeferredIncomeTaxLiabilities / dei_LegalEntityAxis = lnt_WplMember | 1,055.4 us-gaap_DeferredIncomeTaxLiabilities / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | 932.5 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / dei_LegalEntityAxis = lnt_WplMember | 853.8 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / dei_LegalEntityAxis = lnt_WplMember
Current deferred tax assets | (37.5)us-gaap_DeferredTaxAssetsNetCurrent / dei_LegalEntityAxis = lnt_WplMember | (43.3)us-gaap_DeferredTaxAssetsNetCurrent / dei_LegalEntityAxis = lnt_WplMember
Non-current deferred tax liabilities | 970.0 us-gaap_DeferredTaxLiabilitiesNoncurrent / dei_LegalEntityAxis = lnt_WplMember | 897.1 us-gaap_DeferredTaxLiabilitiesNoncurrent / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, net of deferred tax assets | 932.5 us-gaap_DeferredTaxLiabilities / dei_LegalEntityAxis = lnt_WplMember | 853.8 us-gaap_DeferredTaxLiabilities / dei_LegalEntityAxis = lnt_WplMember
Property [Member] |  |
Deferred tax assets, gross | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember
Deferred tax liabilities, gross | 2,627.8 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember | 2,316.3 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember
Deferred tax assets and liabilities, net | 2,627.8 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember | 2,316.3 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember
Property [Member] | IPL [Member] |  |
Deferred tax assets, gross | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 1,531.0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember | 1,338.1 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | 1,531.0 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember | 1,338.1 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_IplMember
Property [Member] | WPL [Member] |  |
Deferred tax assets, gross | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 964.4 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember | 859.1 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | 964.4 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember | 859.1 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_PropertyMember / dei_LegalEntityAxis = lnt_WplMember
Investment In ATC [Member] |  |
Deferred tax assets, gross | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember
Deferred tax liabilities, gross | 131.6 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember | 120.7 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember
Deferred tax assets and liabilities, net | 131.6 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember | 120.7 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember
Investment In ATC [Member] | WPL [Member] |  |
Deferred tax assets, gross | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 131.6 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember | 120.7 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | 131.6 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember | 120.7 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_InvestmentInAtcMember / dei_LegalEntityAxis = lnt_WplMember
Net operating losses carryforward - state [Member] |  |
Deferred tax assets, gross | (45.7)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember | (35.3)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember
Deferred tax assets and liabilities, net | (45.7)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember | (35.3)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardStateMember
Regulatory liability - IPL's tax benefit riders [Member] |  |
Deferred tax assets, gross | (100.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember | (107.8)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember
Deferred tax assets and liabilities, net | (100.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember | (107.8)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember
Regulatory liability - IPL's tax benefit riders [Member] | IPL [Member] |  |
Deferred tax assets, gross | (100.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember | (107.8)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | (100.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember | (107.8)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_RegulatoryLiabilityIplsTaxBenefitRiderMember / dei_LegalEntityAxis = lnt_IplMember
Federal credit carryforward [Member] |  |
Deferred tax assets, gross | (201.0)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember | (167.8)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember
Deferred tax assets and liabilities, net | (201.0)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember | (167.8)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember
Federal credit carryforward [Member] | IPL [Member] |  |
Deferred tax assets, gross | (67.7)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember | (52.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | (67.7)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember | (52.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_IplMember
Federal credit carryforward [Member] | WPL [Member] |  |
Deferred tax assets, gross | (75.2)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember | (57.1)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | (75.2)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember | (57.1)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_FederalCreditCarryforwardMember / dei_LegalEntityAxis = lnt_WplMember
Net operating losses carryforward - federal [Member] |  |
Deferred tax assets, gross | (332.8)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember | (251.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember
Deferred tax assets and liabilities, net | (332.8)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember | (251.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember
Net operating losses carryforward - federal [Member] | IPL [Member] |  |
Deferred tax assets, gross | (160.6)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember | (111.3)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | (160.6)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember | (111.3)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_IplMember
Net operating losses carryforward - federal [Member] | WPL [Member] |  |
Deferred tax assets, gross | (128.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember | (106.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | (128.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember | (106.9)lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_NetOperatingLossesCarryforwardFederalMember / dei_LegalEntityAxis = lnt_WplMember
Other [Member] |  |
Deferred tax assets, gross | (88.1)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember | (108.9)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember
Deferred tax liabilities, gross | 180.1 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember | 210.7 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember
Deferred tax assets and liabilities, net | 92.0 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember | 101.8 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember
Other [Member] | IPL [Member] |  |
Deferred tax assets, gross | (47.2)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember | (64.0)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax liabilities, gross | 81.9 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember | 103.2 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember
Deferred tax assets and liabilities, net | 34.7 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember | 39.2 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_IplMember
Other [Member] | WPL [Member] |  |
Deferred tax assets, gross | (40.3)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember | (37.6)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax liabilities, gross | 80.9 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember | 75.6 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember
Deferred tax assets and liabilities, net | 40.6 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember | 38.0 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_OtherTaxMember / dei_LegalEntityAxis = lnt_WplMember
Subtotal [Member] |  |
Deferred tax assets, gross | (768.5)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember | (671.7)us-gaap_DeferredTaxAssetsGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember
Deferred tax liabilities, gross | 2,939.5 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember | 2,647.7 us-gaap_DeferredIncomeTaxLiabilities / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember
Deferred tax assets and liabilities, net | $ 2,171.0 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember | $ 1,976.0 lnt_TotalDeferredTaxAssetsAndLiabilitiesGross / lnt_DeferredTaxAssetsLiabilitiesAxis = lnt_SubtotalMember

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- Definition Total deferred tax (assets) and liabilities, Gross + References No definition available. + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Asset -URI http://asc.fasb.org/extlink&oid=6510090 + Details
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- Definition Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards expected to be realized or consumed within one year or operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31917-109318 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31931-109318 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31928-109318 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31958-109318 + Details
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