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Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Purchased power expense [Member]
   
Derivatives, Fair Value [Line Items]    
Unrealized gain/(loss) on derivatives $ 1 $ (4)
Non-utility revenue [Member]
   
Derivatives, Fair Value [Line Items]    
Unrealized gain/(loss) on derivatives $ 45 $ (27)