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Leases (Narrative) (Details) (Sheboygan Falls Energy Facility [Member], WPL [Member], USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2005
lease_renewal_period_option
Sheboygan Falls Energy Facility [Member] | WPL [Member]
  
Leases [Line Items]  
Capital lease, agreement term 20 years
Capital leases, renewal options (in number of renewal periods) 2lnt_CapitalLeasesLeaseRenewalPeriodOptions
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= lnt_SheboyganFallsEnergyFacilityMember
Affiliated lease agreement monthly payments $ 1.3lnt_AffiliatedLeaseAgreementMonthlyPayments
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= lnt_SheboyganFallsEnergyFacilityMember
Basis for lease payments, debt to capital ratio 50.00%lnt_LeasePaymentsBasedOnDebtToCapitalRatio
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= lnt_SheboyganFallsEnergyFacilityMember
Basis for lease payments, return on equity ratio 10.90%lnt_LeasePaymentsBasedOnReturnOnEquity
/ dei_LegalEntityAxis
= lnt_WplMember
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= lnt_SheboyganFallsEnergyFacilityMember
Approved agreement terms, review period from date of the order 5 years
Capital lease asset amortization term 20 years