Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Margin deposits	$ 34	$ 37
CECONY [Member]
Derivatives, Fair Value [Line Items]
Margin deposits	$ 14	$ 18