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Receivables (Narrative) (Details) - IPL [Member] $ in Millions | Dec. 31, 2017 USD ($)
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Maximum [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Limit on cash proceeds to be received from third-party | $ 125.0
Receivables Sold [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Available capacity | $ 91.7

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- Definition Sale of Accounts Receivable Program, Capacity Available + References No definition available. + Details
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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The face amount of accounts receivable that will be sold to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522 + Details
Name: | us-gaap_TradeReceivablesHeldForSaleAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=lnt_ReceivablesSoldMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |