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Benefit Plans (Carrying Value Of Entity's Deferred Compensation Obligations Interest And Equity Accounts) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items] |  |
Carrying value of deferred compensation obligations | $ 17.8 lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts | $ 15.9 lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts
IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Carrying value of deferred compensation obligations | $ 5.2 lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts / dei_LegalEntityAxis = lnt_IplMember | $ 5.2 lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts / dei_LegalEntityAxis = lnt_IplMember

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- Definition Carrying value of entity deferred compensation obligations interest and equity accounts + References No definition available. + Details
Name: | lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |