Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 122.1		$ 66.8
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	155.9		118.2
Unbilled utility revenues	68.1		77.4
Other receivables	0.1		2.8
Receivables sold	224.1		198.4
Less: cash proceeds	100.0	[1]	130.0	[1]
Deferred proceeds	124.1		68.4
Less: allowance for doubtful accounts	2.0		1.6
Fair value of deferred proceeds	122.1		66.8
Outstanding receivables past due	$ 21.0		$ 16.1

[1] Changes in cash proceeds are recorded in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s Condensed Consolidated Statements of Cash Flows.