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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, 2014	Dec. 31, 2013
Company Stock Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Carrying value of deferred compensation obligation	$ 8.9lnt_CarryingValueOfDeferredCompensation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CompanyStockAccountsMember	$ 8.0lnt_CarryingValueOfDeferredCompensation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CompanyStockAccountsMember
Fair market value of deferred compensation obligation	$ 15.9lnt_FairMarketValueOfDeferredCompensation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CompanyStockAccountsMember	$ 11.7lnt_FairMarketValueOfDeferredCompensation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CompanyStockAccountsMember

X

- Definition

Carrying value of deferred compensation

+ References

No definition available.

+ Details

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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

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- Details

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