Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collections reinvested in receivables	$ 491.3	$ 442.3
Credit losses, net of recoveries	$ 1.9	$ 2.1

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

No definition available.

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