Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Receivables [Line Items]
Fair value of deferred proceeds	$ 117.7		$ 177.2
IPL [Member]
Receivables [Line Items]
Receivables sold to third party	197.5
Fair value of deferred proceeds	117.7		177.2
Receivables Sold [Member] | IPL [Member]
Receivables [Line Items]
Customer accounts receivable	144.8		134.8
Unbilled utility revenues	52.5		69.7
Other receivables	0.2		0.1
Receivables sold to third party	197.5		204.6
Less: cash proceeds	74.0	[1]	22.0	[1]
Deferred proceeds	123.5		182.6
Less: allowance for doubtful accounts	5.8		5.4
Fair value of deferred proceeds	117.7		177.2
Outstanding receivables past due	$ 21.7		$ 19.9

[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.