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Receivables (Maximum And Average Outstanding Cash Proceeds) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Receivables [Line Items] |  |
Outstanding aggregate cash proceeds (based on daily outstanding balances) | $ 68.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables | $ 27.6 us-gaap_ProceedsFromSaleAndCollectionOfReceivables
Maximum [Member] |  |
Receivables [Line Items] |  |
Outstanding aggregate cash proceeds (based on daily outstanding balances) | $ 118.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RangeAxis = us-gaap_MaximumMember | $ 75.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition The cash inflow associated with the proceeds from sale and collection of receivables during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 + Details
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