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Benefit Plans (Schedule Of Qualified and Non-qualified Pension And Other Postretirement Benefits Costs (Credits)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Defined benefit pension plans [Member] | IPL [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefits costs (credits) associated with Corporate Services employees | $ 0.4 | $ 0.4 | $ 1.4 | $ 1.4
Defined benefit pension plans [Member] | WPL [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefits costs (credits) associated with Corporate Services employees | 0.4 | 0.4 | 1.1 | 1.0
Other Postretirement Benefits Costs (Credits) [Member] | IPL [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefits costs (credits) associated with Corporate Services employees | (0.1) | 0 | (0.2) | 0.1
Other Postretirement Benefits Costs (Credits) [Member] | WPL [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefits costs (credits) associated with Corporate Services employees | $ (0.1) | $ 0.1 | $ (0.2) | $ 0.1

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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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Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration