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Regulatory Matters (Customer Billing Credits) (Details) (IPL [Member], USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
IPL [Member]
  
Regulatory Liabilities [Line Items]  
Customer billing credits $ 6lnt_CustomerBillingCredits
/ dei_LegalEntityAxis
= lnt_IplMember