Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 203.5		$ 66.8
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold	238.0		198.4
Fair value of deferred proceeds	203.5		66.8
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	151.6		118.2
Unbilled utility revenues	86.2		77.4
Other receivables	0.2		2.8
Receivables sold	238.0		198.4
Less: cash proceeds	29.0	[1]	130.0	[1]
Deferred proceeds	209.0		68.4
Less: allowance for doubtful accounts	5.5		1.6
Fair value of deferred proceeds	203.5		66.8
Outstanding receivables past due	$ 21.5		$ 16.1

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