Receivables (Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	$ 97.1		$ 122.4
Unbilled utility revenues	77.4		65.4
Other receivables	21.7		7.5
Receivables sold	196.2		195.3
Less: cash proceeds	113.0	[1]	140.0	[1]
Deferred proceeds	83.2		55.3
Less: allowance for doubtful accounts	1.5		1.6
Fair value of deferred proceeds	81.7		53.7
Outstanding receivables past due	$ 12.4		$ 15.9

[1] Changes in cash proceeds for the six months ended June 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.