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Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Operating Loss Carryforwards [Line Items]  
Deferred tax assets, net operating losses $ 580us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
IPL [Member]  
Operating Loss Carryforwards [Line Items]  
Deferred tax assets, net operating losses 245us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ dei_LegalEntityAxis
= lnt_IplMember
WPL [Member]  
Operating Loss Carryforwards [Line Items]  
Deferred tax assets, net operating losses 213us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ dei_LegalEntityAxis
= lnt_WplMember
Federal [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 970us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
Deferred tax assets, net operating losses 333us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
Tax carryforwards, tax credits 204us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
Deferred tax assets, tax credits 201us-gaap_TaxCreditCarryforwardDeferredTaxAsset
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
Federal [Member] | IPL [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 468us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_IplMember
Deferred tax assets, net operating losses 161us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_IplMember
Tax carryforwards, tax credits 69us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_IplMember
Deferred tax assets, tax credits 68us-gaap_TaxCreditCarryforwardDeferredTaxAsset
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_IplMember
Federal [Member] | WPL [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 376us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_WplMember
Deferred tax assets, net operating losses 129us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_WplMember
Tax carryforwards, tax credits 77us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_WplMember
Deferred tax assets, tax credits 75us-gaap_TaxCreditCarryforwardDeferredTaxAsset
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ dei_LegalEntityAxis
= lnt_WplMember
State [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 881us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		 [1]
Deferred tax assets, net operating losses 46us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		 [1]
State [Member] | IPL [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 291us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ dei_LegalEntityAxis
= lnt_IplMember		 [2]
Deferred tax assets, net operating losses 16us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ dei_LegalEntityAxis
= lnt_IplMember		 [2]
State [Member] | WPL [Member]  
Operating Loss Carryforwards [Line Items]  
Tax carryforwards, net operating losses 171us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ dei_LegalEntityAxis
= lnt_WplMember		 [3]
Deferred tax assets, net operating losses $ 9us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ dei_LegalEntityAxis
= lnt_WplMember		 [3]
After 2024 [Member]  
Operating Loss Carryforwards [Line Items]  
Net operating loss carryforwards, percentage expiring 98.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring
/ lnt_OperatingLossCarryforwardsExpirationDateAxis
= lnt_After2024Member
After 2024 [Member] | IPL [Member]  
Operating Loss Carryforwards [Line Items]  
Net operating loss carryforwards, percentage expiring 96.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring
/ dei_LegalEntityAxis
= lnt_IplMember
/ lnt_OperatingLossCarryforwardsExpirationDateAxis
= lnt_After2024Member
After 2024 [Member] | WPL [Member]  
Operating Loss Carryforwards [Line Items]  
Net operating loss carryforwards, percentage expiring 99.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring
/ dei_LegalEntityAxis
= lnt_WplMember
/ lnt_OperatingLossCarryforwardsExpirationDateAxis
= lnt_After2024Member
[1] At December 31, 2014, Alliant Energy’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2033 with 98% expiring after 2024.
[2] At December 31, 2014, IPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2031 with 96% expiring after 2024.
[3] At December 31, 2014, WPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2031 with 99% expiring after 2024.