v2.4.0.6

Leases Narrative (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 Alliant Energy and WPL [Member] | Dec. 31, 2005 Sheboygan Falls Energy Facility [Member] WPL [Member] lease_renewal_period_option | Dec. 31, 2012 Synthetic Leases [Member] Alliant Energy and WPL [Member]
Leases [Line Items] |  |  |
Residual value of assets |  |  | $ 4
Operating synthetic leases, remaining term | 3 years |  |
Capital lease, agreement term |  | 20 years |
Capital leases, number of lease renewal period options |  | 2 |
Affiliated lease agreement monthly payments |  | $ 1.3 |
Basis for lease payments, debt to capital ratio |  | 50.00% |
Basis for lease payments, return on equity ratio |  | 10.90% |
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
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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
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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
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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

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- Definition Operating Synthetic Leases Remaining Terms + References No definition available. + Details
Name: | lnt_OperatingSyntheticLeasesRemainingTerms
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Residual Value Of Assets + References No definition available. + Details
Name: | lnt_ResidualValueOfAssets
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration