Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
IPL [Member]
Receivables [Line Items]
Collections reinvested in receivables	$ 442.3	$ 475.3
Credit losses, net of recoveries	$ 2.1	$ 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