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Debt (Schedule Of Unamortized Debt Issuance Costs) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Unamortized debt issuance costs | $ 22.4 us-gaap_UnamortizedDebtIssuanceExpense | $ 19.9 us-gaap_UnamortizedDebtIssuanceExpense
IPL [Member] |  |
Unamortized debt issuance costs | 10.7 us-gaap_UnamortizedDebtIssuanceExpense / dei_LegalEntityAxis = lnt_IplMember | 9.7 us-gaap_UnamortizedDebtIssuanceExpense / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Unamortized debt issuance costs | $ 10.8 us-gaap_UnamortizedDebtIssuanceExpense / dei_LegalEntityAxis = lnt_WplMember | $ 9.0 us-gaap_UnamortizedDebtIssuanceExpense / dei_LegalEntityAxis = lnt_WplMember

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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
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Data Type: | na
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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
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Data Type: | na
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