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Asset Retirement Obligations (Asset Retirement Obligations Expected To Be Recognized) (Details) (Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified | 3 Months Ended
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Jun. 30, 2015
Asset Retirement Obligations [Line Items] |
Expected ARO increase not to exceed | $ 85 us-gaap_AssetRetirementObligationPeriodIncreaseDecrease
IPL [Member] |
Asset Retirement Obligations [Line Items] |
Expected ARO increase not to exceed | 75 us-gaap_AssetRetirementObligationPeriodIncreaseDecrease / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
WPL [Member] |
Asset Retirement Obligations [Line Items] |
Expected ARO increase not to exceed | $ 10 us-gaap_AssetRetirementObligationPeriodIncreaseDecrease / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember

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- Definition Net Increase or Decrease in the asset retirement obligation during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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