Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 8.6	$ 6.4
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	4.7	2.8
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	1.2	1.3
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	3.5	1.5
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 3.9	$ 3.6