Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Production tax credits net of state tax impacts	$ 7.5	$ 2.7	$ 14.3	$ 5.5
WPL [Member] | Cedar Ridge Wind Project [Member]
Production tax credits net of state tax impacts	1.1	0.9	2.6	2.0
IPL [Member] | Whispering Willow-East Wind Project [Member]
Production tax credits net of state tax impacts	3.6	1.8	6.4	3.5
WPL [Member] | Bent Tree - Phase I Wind Project [Member]
Production tax credits net of state tax impacts	2.8		5.3
WPL [Member]
Production tax credits net of state tax impacts	$ 3.9	$ 0.9	$ 7.9	$ 2.0