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