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Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Receivables [Line Items]
Collections reinvested in receivables	$ 434.1	$ 422.2	$ 935.3	$ 862.4
Write-off losses (recoveries), net	$ 2.3	$ (0.7)	$ 6.9	$ (0.3)

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- Definition

Collections reinvested related to sales of accounts receivable program.

+ References

No definition available.

+ Details

Name:	lnt_CollectionsReinvestedInReceivables
Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Definition

Credit losses, net of recoveries related to sales of accounts receivable program.

+ References

No definition available.

+ Details

Name:	lnt_CreditLossesNetOfRecoveries
Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

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- Details

Name:	dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: