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Variable Interest Entities (VIEs) (Details) (Riverside PPA [Member], USD $)
In Millions
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Riverside PPA [Member]
    
Costs, excluding fuel costs$ 28.5$ 28.2$ 55.4$ 55.1