Summary Of Significant Accounting Policies (Carrying Value Of Assets Purchased) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Mar. 31, 2011 WPL's Edgewater Unit 5 Purchase [Member] WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Electric plant in service	$ 8,165.4	$ 7,676.8		$ 3,481.4	$ 3,114.6	$ 84.0
Accumulated depreciation (accum. depr.)	(3,206.0)	(2,982.2)		(1,372.2)	(1,243.8)	(50.0)
CWIP						2
Production fuel	101.9	122.8		34.2	42.7	1.0
Materials and supplies	58.5	61.6		25.7	25.7	1.0
Carrying value of assets purchased	$ 608.1	$ 833.3	$ 1,149.6			$ 38.0