Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
IPL [Member]
Receivables [Line Items]
Collections reinvested in receivables	$ 1,795.7	$ 1,354.2
Credit losses, net of recoveries	$ 10.9	$ 7.9

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