Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 86.9		$ 66.8
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	158.8		118.2
Unbilled utility revenues	70.9		77.4
Other receivables	0.1		2.8
Receivables sold	229.8		198.4
Less: cash proceeds	140.0	[1]	130.0	[1]
Deferred proceeds	89.8		68.4
Less: allowance for doubtful accounts	2.9		1.6
Fair value of deferred proceeds	86.9		66.8
Outstanding receivables past due	$ 22.0		$ 16.1

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