Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Alliant Energy Corporation [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 6.4	$ 6.8
IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	3.6	2.8
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	2.8	4.0
Whispering Willow-East Wind Project [Member] | Alliant Energy Corporation [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	3.6	2.8
Whispering Willow-East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	3.6	2.8
Bent Tree - Phase I Wind Project [Member] | Alliant Energy Corporation [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.5	2.5
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.5	2.5
Cedar Ridge Wind Project [Member] | Alliant Energy Corporation [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.3	1.5
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 1.3	$ 1.5