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Leases (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2020 lease_renewal_period_option
Sheboygan Falls Energy Facility [Member] | WPL [Member] |
Leases [Line Items] |
Finance lease, renewal options (in number of renewal periods) | 2

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- Definition Capital Leases, Lease Renewal Period Options + References No definition available. + Details
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Data Type: | xbrli:integerItemType
Balance Type: | na
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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