Variable Interest Entities (VIEs) (Details) (Riverside Energy Center PPA [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 MW	Sep. 30, 2011	Apr. 30, 2012 Public Service Commission Of Wisconsin [Member]
Variable Interest Entity [Line Items]
Costs, excluding fuel costs	$ 29.7	$ 28.5	$ 57.7	$ 55.4
Approval of business acquisition purchase price					$ 393
Amount of electricity output (in megawatts)			490