Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Production Tax Credits [Line Items]
Production tax credits	$ 9.4	$ 10.0
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits	5.1	5.4
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Nameplate Capacity (in megawatts)	68
Production tax credits	1.2	1.2
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Nameplate Capacity (in megawatts)	201
Production tax credits	3.9	4.2
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Nameplate Capacity (in megawatts)	200
Production tax credits	$ 4.3	$ 4.6