Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	$ 31.3	$ 31.0	$ 24.8
IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	14.0	14.4	12.5
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	17.3	16.6	12.3
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	13.3	12.5	8.3
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	14.0	14.4	12.5
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Wind EGU capacity (in megawatts)	201
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Wind EGU capacity (in megawatts)	68
Production tax credits, net of state tax impacts	$ 4.0	$ 4.1	$ 4.0
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Wind EGU capacity (in megawatts)	200