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Summary Of Significant Accounting Policies (Narrative) (Details) (Minimum [Member])
Mar. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]  
Debt issuance costs percentage, on total long-term debt 1.00%lnt_DebtIssuanceCostsPercentageOnLongtermDebt
IPL [Member]
  
New Accounting Pronouncements or Change in Accounting Principle [Line Items]  
Debt issuance costs percentage, on total long-term debt 1.00%lnt_DebtIssuanceCostsPercentageOnLongtermDebt
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
WPL [Member]
  
New Accounting Pronouncements or Change in Accounting Principle [Line Items]  
Debt issuance costs percentage, on total long-term debt 1.00%lnt_DebtIssuanceCostsPercentageOnLongtermDebt
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember