Derivative Instruments and Hedging Activities - Aggregate Fair Value of All Derivative Instruments with Credit-Risk-Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	$ 19
Collateral posted
Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	32
Additional collateral
Additional collateral aggregate fair value down below investment grade [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	38
Additional collateral	22
CECONY [Member]
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	14
Collateral posted
CECONY [Member] | Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	15
Additional collateral
CECONY [Member] | Additional collateral aggregate fair value down below investment grade [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	1
Additional collateral	$ 15