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Summary Of Significant Accounting Policies (Schedule Of Allowance For Funds Used During Construction Recovery Rate) (Details) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
IPL [Member] | IPL (FERC formula - Marshalltown CWIP) [Member] |  |  |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Recovery rates | 8.00%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaMarshalltownProjectMember | [1] | 0.00%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaMarshalltownProjectMember | [1] | 0.00%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaMarshalltownProjectMember | [1]
IPL [Member] | IPL (FERC formula - other CWIP) [Member] |  |  |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Recovery rates | 7.80%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaOtherProjectsMember |  | 8.20%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaOtherProjectsMember |  | 8.20%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_IplFercFormulaOtherProjectsMember |
IPL [Member] | Marshalltown Generating Station [Member] |  |  |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Return on common equity for calculation of AFUDC | 10.30%lnt_ReturnOnCommonEquityForCalculationOfAllowanceForFundsUsedDuringConstruction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |  |  |  |
WPL [Member] | WPL (PSCW formula - retail jurisdiction) [Member] |  |  |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Recovery rates | 8.20%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplPscwFormulaRetailJurisdictionMember |  | 8.20%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplPscwFormulaRetailJurisdictionMember |  | 8.80%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplPscwFormulaRetailJurisdictionMember |
WPL [Member] | WPL (FERC formula - wholesale jurisdiction) [Member] |  |  |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Recovery rates | 4.10%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplFercFormulaWholesaleJurisdictionMember |  | 4.50%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplFercFormulaWholesaleJurisdictionMember |  | 7.90%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_WplFercFormulaWholesaleJurisdictionMember |
[1] | In 2013, the IUB issued an order establishing rate-making principles for Marshalltown that requires a 10.3% return on common equity for the calculation of AFUDC related to the construction of such facility.

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- Definition Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430 + Details
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