v2.4.0.6

Summary Of Significant Accounting Policies (Carrying Value Of Assets Purchased) (Details) (Riverside Energy Center [Member], WPL [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2012
--- | ---
Riverside Energy Center [Member] | WPL [Member] |
Public Utility, Property, Plant and Equipment [Line Items] |
Electric plant in service | $ 512.7
Accumulated depreciation | (121.4)
Current assets | 4.2
Other assets | 8.0
Carrying value of assets purchased | $ 403.5

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets + References No definition available. + Details
Name: | lnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets + References No definition available. + Details
Name: | lnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Public Utilities, Plant In Service, Accumulated Depreciation + References No definition available. + Details
Name: | lnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPublicUtilitiesPlantInServiceAccumulatedDepreciation
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Public Utilities, Plant In Service, Gross + References No definition available. + Details
Name: | lnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPublicUtilitiesPlantInServiceGross
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Net amount recognized for aggregate assets, including goodwill, in excess of or less than the aggregate liabilities assumed (also referred to as net assets acquired). This amount may also be viewed as incremental amount of equity that the consolidated entity (including the portion attributable to a noncontrolling interest) will recognize as a result of the business combination. + References No definition available. + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_PublicUtilityPropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration