Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 32.9		$ 53.7
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	116.1		122.4
Unbilled utility revenues	48.7		65.4
Other receivables	14.8		7.5
Receivables sold	179.6		195.3
Less: cash proceeds	145.0	[1]	140.0	[1]
Deferred proceeds	34.6		55.3
Less: allowance for doubtful accounts	1.7		1.6
Fair value of deferred proceeds	32.9		53.7
Outstanding receivables past due	$ 18.0		$ 15.9

[1] Changes in cash proceeds during the first quarter of 2012 are recorded in "Sales of accounts receivable" in operating activities in Alliant Energy's and IPL's Condensed Consolidated Statements of Cash Flows.