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Benefit Plans (Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Pension Plans, Defined Benefit [Member] | IPL [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Allocated qualified and non-qualified costs (credits) | $ 1.4 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | $ 4.8 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | $ 4.9 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Pension Plans, Defined Benefit [Member] | WPL [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Allocated qualified and non-qualified costs (credits) | 1.1 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | 3.6 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | 3.6 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Allocated qualified and non-qualified costs (credits) | (0.3)lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | (0.3)lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 0.1 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Allocated qualified and non-qualified costs (credits) | $ (0.2)lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | $ (0.2)lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | $ 0.1 lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
[1] | Refer to IPL’s and WPL’s “Net Periodic Benefit Costs (Credits)” tables above for additional benefits costs related to the Cash Balance Plan allocated to IPL and WPL by Corporate Services in 2013.

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- Definition Pension and other postretirement benefits plans costs allocated from Corporate Services. + References No definition available. + Details
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