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Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Derivatives, Fair Value [Line Items]    
Margin deposits $ 33 $ 37
CECONY [Member]
   
Derivatives, Fair Value [Line Items]    
Margin deposits $ 14 $ 18