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Regulatory Matters (Tax Benefit Riders) (Details) (Alliant Energy and IPL [Member], USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Electric tax benefit rider [Member] |
Regulatory Liabilities [Line Items] |
Decrease in regulatory liabilities | $ 18.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_ElectricTaxBenefitRiderMember
Gas tax benefit rider [Member] |
Regulatory Liabilities [Line Items] |
Decrease in regulatory liabilities | 3.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_GasTaxBenefitRiderMember
Tax benefit riders [Member] |
Regulatory Liabilities [Line Items] |
Decrease in regulatory liabilities | $ 21.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember

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- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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