Asset Sales, Divestitures and Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Continuing Operations Attributable to Parent	$ 119.3	$ 98.0	$ 291.4	$ 268.9
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	0	11.5	0	11.5
Net Income (Loss) Attributable to Parent	119.3	109.5	291.4	280.4
Asset Sales, Divestitures and Discontinued Operations (Textuals)
Ownership percentage in Edgewater Generating Unit 5 sold				25.00%
Proceeds from the sale of interest in Edgewater Generating Unit				$ 38