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Acquisitions and Divestitures Sale of Riverside Energy Center (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
MW
Dec. 31, 2011
MW
Sep. 30, 2012
Riverside Energy Center [Member]
MW
Business Acquisition [Line Items]      
Power generation capacity 10,500 11,391 603
Proceeds from Sale of Productive Assets     $ 392
Assets 16,593 17,371 436
Liabilities 12,479 13,007 1
Working capital     $ 42