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Regulatory Matters (Customer Billing Credits) (Details) - USD ($) $ in Millions | 12 Months Ended | 36 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2016
IPL [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Billing credits to reduce retail electric customers' bills | $ 9 | $ 24 | $ 72 | $ 105

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- Definition Customer Billing Credits + References No definition available. + Details
Name: | lnt_CustomerBillingCredits
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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_RegulatoryLiabilitiesLineItems
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: |