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Debt (Schedule Of Unamortized Debt Issuance Costs) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Unamortized debt issuance costs $ 22.4us-gaap_UnamortizedDebtIssuanceExpense $ 19.9us-gaap_UnamortizedDebtIssuanceExpense
IPL [Member]    
Unamortized debt issuance costs 10.7us-gaap_UnamortizedDebtIssuanceExpense
/ dei_LegalEntityAxis
= lnt_IplMember		 9.7us-gaap_UnamortizedDebtIssuanceExpense
/ dei_LegalEntityAxis
= lnt_IplMember
WPL [Member]    
Unamortized debt issuance costs $ 10.8us-gaap_UnamortizedDebtIssuanceExpense
/ dei_LegalEntityAxis
= lnt_WplMember		 $ 9.0us-gaap_UnamortizedDebtIssuanceExpense
/ dei_LegalEntityAxis
= lnt_WplMember