Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Alliant Energy [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 6.8	$ 7.5	$ 13.2	$ 14.3
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	3.7	3.9	6.5	7.9
Cedar Ridge [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.0	1.1	2.3	2.6
Bent Tree - Phase I [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	2.7	2.8	4.2	5.3
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 3.1	$ 3.6	$ 6.7	$ 6.4