Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	$ 33.1	$ 31.3	$ 31.0
IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	14.5	13.8	14.1
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	18.6	17.5	16.9
Cedar Ridge Wind Farm [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	4.5	4.1	4.2
Bent Tree Wind Farm [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	14.1	13.4	12.7
Whispering Willow - East Wind Farm [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	$ 14.5	$ 13.8	$ 14.1
Cedar Ridge Wind Farm [Member] | WPL [Member]
Production Tax Credits [Line Items]
Nameplate capacity (in megawatts) | MW	68
Bent Tree Wind Farm [Member] | WPL [Member]
Production Tax Credits [Line Items]
Nameplate capacity (in megawatts) | MW	201
Whispering Willow - East Wind Farm [Member] | IPL [Member]
Production Tax Credits [Line Items]
Nameplate capacity (in megawatts) | MW	200