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Leases (Narrative) (Details) (Sheboygan Falls Energy Facility [Member], WPL [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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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) | 2 lnt_CapitalLeasesLeaseRenewalPeriodOptions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember
Affiliated lease agreement monthly payments | $ 1.3 lnt_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

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- Definition Affiliated Lease Agreement Monthly Payments + References No definition available. + Details
Name: | lnt_AffiliatedLeaseAgreementMonthlyPayments
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Capital Lease Asset Amortization Term + References No definition available. + Details
Name: | lnt_CapitalLeaseAssetAmortizationTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Capital Leases, Agreement Term + References No definition available. + Details
Name: | lnt_CapitalLeasesAgreementTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Capital Leases, Approved Agreement Terms, Review Period From Date of the Order + References No definition available. + Details
Name: | lnt_CapitalLeasesApprovedAgreementTermsReviewPeriodFromDateOfOrder
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Capital Leases, Lease Renewal Period Options + References No definition available. + Details
Name: | lnt_CapitalLeasesLeaseRenewalPeriodOptions
Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Lease Payments Based On Debt To Capital Ratio + References No definition available. + Details
Name: | lnt_LeasePaymentsBasedOnDebtToCapitalRatio
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Lease Payments Based On Return On Equity + References No definition available. + Details
Name: | lnt_LeasePaymentsBasedOnReturnOnEquity
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | lnt_LeasesLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration