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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 5.1	$ 4.5	$ 22.4	$ 17.7
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	2.8	2.5	12.1	9.0
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	0.6	0.7	2.9	3.0
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	2.2	1.8	9.2	6.0
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits (net of state tax impacts)	$ 2.3	$ 2.0	$ 10.3	$ 8.7