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Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Receivables [Line Items]
Collections reinvested in receivables	$ 480.1	$ 520.1	$ 1,403.1	$ 1,537.3
Credit losses, net of recoveries	$ 3.3	$ 6.4	$ 6.8	$ 12.8

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- Definition

Collections reinvested related to sales of accounts receivable program.

+ References

No definition available.

+ Details

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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: