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	Mar. 31, 2015
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	$ 58
Collateral posted	5
Downgrade to Below Investment Grade from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	73
CECONY [Member]
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	40
CECONY [Member] | Downgrade to Below Investment Grade from Current Ratings [Member]
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	$ 46