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Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2014 counterparty
Derivative [Line Items] |
Cash collateral posted by counterparty | $ 4.9
Number of counterparties exceeding derivative asset position (in counterparties) | 1

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- Definition Number of Counterparties Exceeding Derivative Asset Position + References No definition available. + Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959 + Details
Name: | us-gaap_CollateralAlreadyPostedAggregateFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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