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Benefit Plans (Regulatory Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | $ 38.0 | $ 38.2
WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | $ 29.5 | $ 28.0

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- Definition Defined Benefit Plan Regulatory Asset Allocated From Corporate Services + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanRegulatoryAssetAllocatedFromCorporateServices
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |