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Derivative Instruments (Effect of Derivative Instruments on Consolidated Statements of Income) (Details) (USD $)
In Millions
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2010
Gain/(Loss) Recognized in Income on Derivative Instruments $ (1) $ (2) $ 6
Commodity Contracts [Member] | Operating Revenue [Member]
     
Gain/(Loss) Recognized in Income on Derivative Instruments (1) (2) 3
Commodity Contracts [Member] | Fuel for Electric Generation [Member]
     
Gain/(Loss) Recognized in Income on Derivative Instruments     2
Commodity Contracts [Member] | Purchased And Interchange Power [Member]
     
Gain/(Loss) Recognized in Income on Derivative Instruments     $ 1