Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Production tax credits net of state tax impacts	$ 3.8	$ 1.4	$ 18.1	$ 6.9
WPL [Member] | Cedar Ridge Wind Project [Member]
Production tax credits net of state tax impacts	0.6	0.4	3.2	2.4
IPL [Member] | Whispering Willow-East Wind Project [Member]
Production tax credits net of state tax impacts	1.8	1.0	8.2	4.5
WPL [Member] | Bent Tree - Phase I Wind Project [Member]
Production tax credits net of state tax impacts	1.4		6.7
WPL [Member]
Production tax credits net of state tax impacts	$ 2.0	$ 0.4	$ 9.9	$ 2.4