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, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Margin deposits	$ 8	$ 17
CECONY [Member]
Derivatives, Fair Value [Line Items]
Margin deposits	$ 8	$ 16