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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Income Taxes [Line Items] |
Increase (decrease) non-current deferred tax liabilities | $ 45.8 lnt_IncreaseDecreaseInDeferredIncomeTaxesNoncurrentLiabilities
IPL [Member] |
Income Taxes [Line Items] |
Increase (decrease) non-current deferred tax liabilities | 29.0 lnt_IncreaseDecreaseInDeferredIncomeTaxesNoncurrentLiabilities / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Income Taxes [Line Items] |
Increase (decrease) non-current deferred tax liabilities | $ 14.4 lnt_IncreaseDecreaseInDeferredIncomeTaxesNoncurrentLiabilities / dei_LegalEntityAxis = lnt_WplMember

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Period Type: |

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