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Regulatory Matters (Revenue Requirement Adjustment) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Regulatory Liabilities [Line Items] |  |  |
Electric revenues | $ 2,713.6 us-gaap_ElectricDomesticRegulatedRevenue | $ 2,689.0 us-gaap_ElectricDomesticRegulatedRevenue | $ 2,589.3 us-gaap_ElectricDomesticRegulatedRevenue
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Electric revenues | $ 15.0 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_RevenueRequirementAdjustmentMember | $ 24.0 us-gaap_ElectricDomesticRegulatedRevenue / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_RevenueRequirementAdjustmentMember |

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- Definition Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(b)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_ElectricDomesticRegulatedRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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