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, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Margin deposits	$ 37	$ 27
CECONY [Member]
Derivatives, Fair Value [Line Items]
Margin deposits	$ 35	$ 25