Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds	$ 170.8		$ 203.5
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable	183.9		151.6
Unbilled utility revenues	67.6		86.2
Other receivables	0.1		0.2
Receivables sold	251.6		238.0
Less: cash proceeds	75.0	[1]	29.0	[1]
Deferred proceeds	176.6		209.0
Less: allowance for doubtful accounts	5.8		5.5
Fair value of deferred proceeds	170.8		203.5
Outstanding receivables past due	$ 27.4		$ 21.5

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