Derivative Instruments and Hedging Activities - Aggregate Fair Value of Companies' Derivative Instruments with Credit-Risk-Related Contingent Features (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Collateral posted	$ 1
Downgrade to Below Investment Grade from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net asset position additional collateral	15
Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	16
Additional Collateral Required Due To Loss Of Unsecured Credit [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	$ 3