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v3.8.0.1

Fair Value Measurements (Narrative) (Details) | 9 Months Ended
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Sep. 30, 2017
IPL [Member] |
Cumulative preferred stock rate | 5.10%

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- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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