Receivables (Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	$ 137.9		$ 122.4
Unbilled utility revenues	64.7		65.4
Other receivables	8.3		7.5
Receivables sold	210.9		195.3
Less: cash proceeds	55.0	[1]	140.0	[1]
Deferred proceeds	155.9		55.3
Less: allowance for doubtful accounts	2.0		1.6
Fair value of deferred proceeds	153.9		53.7
Outstanding receivables past due	$ 16.6		$ 15.9

[1] Changes in cash proceeds for the nine months ended September 30, 2012 are recorded in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s Condensed Consolidated Statements of Cash Flows.