Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Alliant Energy [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 4.5	$ 3.8	$ 17.7	$ 18.1
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	2.5	2.0	9.0	9.9
Cedar Ridge [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	0.7	0.6	3.0	3.2
Bent Tree - Phase I [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.8	1.4	6.0	6.7
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 2.0	$ 1.8	$ 8.7	$ 8.2