Receivables (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2015		Dec. 31, 2014
Receivables [Line Items]
Deferred proceeds		$ 117.7		$ 177.2
IPL [Member]
Receivables [Line Items]
Receivables sold to third party		197.5
Deferred proceeds		117.7		177.2
Whiting Petroleum Corporation [Member]
Receivables [Line Items]
Final payment of certain future tax benefits expected to be realized	26
Receivables Sold [Member] | IPL [Member]
Receivables [Line Items]
Receivables sold to third party		197.5		204.6
Cash proceeds of receivables sold		74.0	[1]	22.0	[1]
Deferred proceeds		$ 117.7		$ 177.2

[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.