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Receivables (Narrative) (Details) - IPL [Member] - USD ($) $ in Millions | Apr. 30, 2018 | Mar. 31, 2018
--- | --- | ---
Receivables Sold [Member] |  |
Receivables [Line Items] |  |
Available capacity |  | $ 0.0
Outstanding receivables past due |  | $ 50.7
Subsequent Event [Member] | Minimum [Member] |  |
Receivables [Line Items] |  |
Limit on cash proceeds | $ 90.0 |
Subsequent Event [Member] | Maximum [Member] |  |
Receivables [Line Items] |  |
Limit on cash proceeds | $ 110.0 |

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- Definition Outstanding receivables past due + 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 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=82891408&loc=d3e4428-111522 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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