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Regulatory Matters (IPL's Tax Benefit Riders) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Regulatory Assets [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | $ 10.8 us-gaap_IncreaseDecreaseInRegulatoryLiabilities | $ (90.8)us-gaap_IncreaseDecreaseInRegulatoryLiabilities | $ 16.4 us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | (85.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_ElectricTaxBenefitRiderMember |  |
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | (12.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_GasTaxBenefitRiderMember |  |
Alliant Energy and IPL [Member] | Tax accounting method changes [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | 75.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_TaxAccountingMethodChangeMember |  |
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | $ (22.0)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
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- Details
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Period Type: | duration

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- Details
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