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Investments (Cash Surrender Value Of Life Insurance Policies) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Cash surrender value $ 47.0us-gaap_CashSurrenderValueOfLifeInsurance $ 46.5us-gaap_CashSurrenderValueOfLifeInsurance
IPL [Member]    
Cash surrender value 18.1us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= lnt_IplMember		 17.3us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= lnt_IplMember
WPL [Member]    
Cash surrender value $ 11.4us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= lnt_WplMember		 $ 12.3us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= lnt_WplMember