Variable Interest Entities (VIEs) (Details) (Riverside Energy Center PPA [Member], USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2012 MW	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Variable Interest Entity [Line Items]
Costs, excluding fuel costs		$ 21.7	$ 20.5	$ 28.0	$ 26.9
Amount of electricity output (in megawatts)	490
Public Service Commission Of Wisconsin [Member]
Variable Interest Entity [Line Items]
Approval of business acquisition purchase price	393