Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	$ 24.8		$ 27.1		$ 11.2
IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	12.5		12.3		7.7
WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	12.3		14.8		3.5
Whispering Willow - East Wind Project [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	12.5		12.3		7.7
Whispering Willow - East Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	12.5		12.3		7.7
Whispering Willow - East Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	0		0		0
Bent Tree - Phase I Wind Project [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	9.3	[1]	9.3	[1]	1.2	[1]
Bent Tree - Phase I Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	0	[1]	0	[1]	0	[1]
Bent Tree - Phase I Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	9.3	[1]	9.3	[1]	1.2	[1]
Cedar Ridge Wind Project [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	4.0		4.5		3.3
Cedar Ridge Wind Project [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	0		0		0
Cedar Ridge Wind Project [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	4.0		4.5		3.3
Subtotal [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	25.8		26.1		12.2
Subtotal [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	12.5		12.3		7.7
Subtotal [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	13.3		13.8		4.5
Deferral [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	(1.0)	[1]	1.0	[1]	(1.0)	[1]
Deferral [Member] | IPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	0	[1]	0	[1]	0	[1]
Deferral [Member] | WPL [Member]
Production Tax Credits [Line Items]
Production tax credits, net of state tax impacts	$ (1.0)	[1]	$ 1.0	[1]	$ (1.0)	[1]

[1] In 2010 and 2012, WPL deferred the retail portion of the production tax credits generated in 2010 from its Bent Tree - Phase I wind project pursuant to orders issued by the PSCW in December 2009 and July 2012, respectively. As a result of a regulatory assessment completed in 2011, the retail portion of the production tax credit deferral recorded in 2010 was reversed.