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Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Alliant Energy and IPL [Member] | Tax benefit riders [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Decrease in regulatory liabilities | $ 21.0 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_TaxBenefitRiderMember |  |
WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Decrease in regulatory liabilities | 5.1 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember | 26.1 us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_WplMember |
2014 Test Year Retail Electric Fuel Related Request [Member] | WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Annual bandwidth for fuel-related costs |  |  | 2.00%lnt_RateOfFuelCostsCapacity / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember
2014 Test Year Retail Electric Fuel Related Request [Member] | Alliant Energy and WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Under-collection of retail fuel-related costs |  |  | 33 lnt_OverCollectionOfRetailFuelRelatedCosts / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember
Fuel-related costs | $ 28 us-gaap_DeferredFuelCost / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember |  |

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- Definition Rate of fuel costs capacity + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378 + Details
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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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