Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 66.8		$ 53.7
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables sold	198.4		195.3
Fair value of deferred proceeds	66.8		53.7
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	118.2		122.4
Unbilled utility revenues	77.4		65.4
Other receivables	2.8		7.5
Receivables sold	198.4		195.3
Less: cash proceeds	130.0	[1]	140.0	[1]
Deferred proceeds	68.4		55.3
Less: allowance for doubtful accounts	1.6		1.6
Fair value of deferred proceeds	66.8		53.7
Outstanding receivables past due	$ 16.1		$ 15.9

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