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Benefit Plans (Regulatory Assets and Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | $ 26.5 | $ 38.1
Regulatory liabilities | 1.7 | 0.6
WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | 19.8 | 25.5
Regulatory liabilities | $ 1.3 | $ 0.4

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- Definition Defined Benefit Plan Regulatory Asset Allocated From Corporate Services + References No definition available. + Details
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Namespace Prefix: | lnt_
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Balance Type: | debit
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- Definition Defined Benefit Plan Regulatory Liabilities Allocated From Corporate Services + References No definition available. + Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration