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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 $ 21.0us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndIplMember
/ us-gaap_RegulatoryLiabilityAxis
= lnt_TaxBenefitRiderMember		    
WPL [Member]      
Regulatory Matters [Line Items]      
Decrease in regulatory liabilities 5.1us-gaap_IncreaseDecreaseInRegulatoryLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember		 26.1us-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     33lnt_OverCollectionOfRetailFuelRelatedCosts
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember
Fuel-related costs $ 28us-gaap_DeferredFuelCost
/ dei_LegalEntityAxis
= lnt_AlliantEnergyAndWplMember
/ us-gaap_PublicUtilitiesRegulatoryProceedingAxis
= lnt_TwoThousandFourteenTestYearRetailElectricFuelRelatedRequestMember