Derivative Instruments and Hedging Activities - Aggregate Fair Value of All Derivative Instruments with Credit-Risk-Related Contingent Features (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 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	28
Downgrade to Below Investment Grade from Current Ratings [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net asset position additional collateral	1
Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	17
Additional Collateral Required Due To Loss Of Unsecured Credit [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	$ 5