v2.4.0.6

Benefit Plans (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | $ 38.1 | $ 33.7
Regulatory liabilities | 0.6 | 0.3
WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Regulatory assets | 25.5 | 22.3
Regulatory liabilities | $ 0.4 | $ 0.2

X
- Definition Defined Benefit Plan Regulatory Asset Allocated From Corporate Services + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanRegulatoryAssetAllocatedFromCorporateServices
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Defined Benefit Plan Regulatory Liabilities Allocated From Corporate Services + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanRegulatoryLiabilitiesAllocatedFromCorporateServices
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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