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Derivative Instruments and Hedging Activities (Income Statement Impact of Derivative Instruments) (Details) (Economic Hedges, USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Economic Hedges
       
Derivative [Line Items]        
Realized gains (losses) $ (77) $ (58) $ (199) $ (132)
Unrealized gains (losses) $ (91) $ (110) $ (29) $ (433)