v2.4.0.6

Benefit Plans (Carrying Value And Fair Market Value Of The Deferred Compensation Obligation for Company Stock Accounts) (Details) (Company Stock Accounts [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011
Company Stock Accounts [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Carrying value of deferred compensation obligation | $ 7.3 | $ 8.3
Fair market value of deferred compensation obligation | $ 9.5 | $ 11.6

X
- Definition Carrying value of deferred compensation + References No definition available. + Details
Name: | lnt_CarryingValueOfDeferredCompensation
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Fair market value of deferred compensation + References No definition available. + Details
Name: | lnt_FairMarketValueOfDeferredCompensation
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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