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Investments (Cash Surrender Value Of Life Insurance Policies) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Cash surrender value | $ 47.0 us-gaap_CashSurrenderValueOfLifeInsurance | $ 46.5 us-gaap_CashSurrenderValueOfLifeInsurance
IPL [Member] |  |
Cash surrender value | 18.1 us-gaap_CashSurrenderValueOfLifeInsurance / dei_LegalEntityAxis = lnt_IplMember | 17.3 us-gaap_CashSurrenderValueOfLifeInsurance / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Cash surrender value | $ 11.4 us-gaap_CashSurrenderValueOfLifeInsurance / dei_LegalEntityAxis = lnt_WplMember | $ 12.3 us-gaap_CashSurrenderValueOfLifeInsurance / dei_LegalEntityAxis = lnt_WplMember

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- Definition Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 4 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6812-107765 + Details
Name: | us-gaap_CashSurrenderValueOfLifeInsurance
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
Balance Type: |
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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
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