Variable Interest Entities (VIEs) (Details) (Riverside Energy Center PPA [Member], USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
	Apr. 30, 2012 MW	Mar. 31, 2012	Mar. 31, 2011
Variable Interest Entity [Line Items]
Costs, excluding fuel costs		$ 6.3	$ 6.4
Amount of electricity output	490
Public Service Commission Of Wisconsin [Member]
Variable Interest Entity [Line Items]
Business acquisition date approval	April 2012
Business acquisition cost of acquired entity purchase price approved	393