Variable Interest Entities (VIEs) (Details) (Riverside Energy Center (Riverside) PPA [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Riverside Energy Center (Riverside) PPA [Member]
Variable Interest Entity [Line Items]
Costs, excluding fuel costs	$ 62.0	$ 61.0	$ 63.0