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Regulatory Matters (IPL's Tax Benefit Riders) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Regulatory Assets [Line Items]      
Increase (decrease) in regulatory liabilities $ 10.8us-gaap_IncreaseDecreaseInRegulatoryLiabilities $ (90.8)us-gaap_IncreaseDecreaseInRegulatoryLiabilities $ 16.4us-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 75.0us-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