Derivative Instruments and Hedging Activities - Aggregate Fair Value of Companies' Derivative Instruments with Credit-Risk-Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	$ 78
Collateral posted	1
Downgrade One Level from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	6
Downgrade to Below Investment Grade from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	105
CECONY [Member]
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	58
CECONY [Member] | Downgrade One Level from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	2
CECONY [Member] | Downgrade to Below Investment Grade from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	$ 63