Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions	9 Months Ended			9 Months Ended		12 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 IPL [Member] Receivables Sold [Member]		Dec. 31, 2010 IPL [Member] Receivables Sold [Member]		Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Dec. 31, 2010 IPL [Member]
Customer accounts receivable				$ 139.8		$ 133.0
Unbilled utility revenues				54.8		80.9
Other receivables				9.7		5.7
Receivables sold				204.3		219.6		204.3		219.6
Less: cash proceeds	(50.0)	(100.0)		115.0	[1]	65.0	[1]	(50.0)	(100.0)
Deferred proceeds	87.4		152.9	89.3		154.6		87.4		152.9
Less: allowance for doubtful accounts				1.9		1.7
Fair value of deferred proceeds				87.4		152.9
Outstanding receivables past due				$ 16.4		$ 14.1

[1] Changes in cash proceeds during the nine months ended Sep. 30, 2011 are recorded in "Sales of accounts receivable" in operating activities in Alliant Energy's and IPL's Condensed Consolidated Statements of Cash Flows.