Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	$ (26.5)	$ 159.2
Allowance for funds used during construction	3.8	3.1
Increase in regulatory liabilities	(26.5)	159.2
Capitalized expenditures	122.1	229.4
WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	2.3	1.7
Capitalized expenditures	65.5	124.8
Edgewater Unit 5 Emission Controls [Member] | Alliant Energy And WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	5.0
Capitalized expenditures	84.0
Tax Benefit Rider [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities		20.0
Increase in regulatory liabilities		20.0
Green Lake And Fond Du Lac Counties Wind Site [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Asset impairment loss		$ 5