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v2.4.1.9

Fair Value Measurements (Narrative) (Details) (IPL [Member]) | 3 Months Ended
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Mar. 31, 2015
IPL [Member] |
Cumulative preferred stock rate | 5.10%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_IplMember

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