v2.4.0.6

Variable Interest Entities (VIEs) (Details) (Riverside Energy Center PPA [Member], USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 1 Months Ended
--- | --- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 MW | Sep. 30, 2011 | Apr. 30, 2012 Public Service Commission Of Wisconsin [Member]
Variable Interest Entity [Line Items] |  |  |  |  |
Costs, excluding fuel costs | $ 29.7 | $ 28.5 | $ 57.7 | $ 55.4 |
Approval of business acquisition purchase price |  |  |  |  | $ 393
Amount of electricity output (in megawatts) |  |  | 490 |  |

X
- Definition Amount Of Electricity Output + References No definition available. + Details
Name: | lnt_AmountOfElectricityOutput
Namespace Prefix: | lnt_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Business Acquisition Cost Of Acquired Entity Purchase Price Approved + References No definition available. + Details
Name: | lnt_BusinessAcquisitionCostOfAcquiredEntityPurchasePriceApproved
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Costs, excluding fuel costs, related to the Riverside Energy Center purchased power agreement + References No definition available. + Details
Name: | lnt_CostsExcludingFuelCostsRelatedToRiversidePpa
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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