v2.4.0.6

Summary Of Significant Accounting Policies (Schedule Of Electric Plant In Service) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service, total | $ 9,070.7 | $ 8,165.4
Generation [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 4,798.9 | 4,100.6
Distribution [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 3,981.5 | 3,782.1
Other [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 290.3 | 282.7
IPL [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service, total | 4,815.2 | 4,684.0
IPL [Member] | Generation [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 2,393.0 | 2,392.3
IPL [Member] | Distribution [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 2,205.9 | 2,074.8
IPL [Member] | Other [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 216.3 | 216.9
WPL [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service, total | 4,255.5 | 3,481.4
WPL [Member] | Generation [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 2,405.9 | 1,708.3
WPL [Member] | Distribution [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | 1,775.6 | 1,707.3
WPL [Member] | Other [Member] |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |
Electric utilities, plant in service | $ 74.0 | $ 65.8

X
- Definition Carrying amount as of the balance sheet date of long-lived, depreciable electric utility plant. + References No definition available. + Details
Name: | lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Period end amount of total gross PPE. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
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