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	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	$ 52
Collateral posted
Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	24
Additional collateral	4
Additional collateral aggregate fair value down below investment grade [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	24
Additional collateral	77
CECONY [Member]
Derivatives, Fair Value [Line Items]
Aggregate fair value - net liabilities	39
Collateral posted
CECONY [Member] | Additional Collateral Required Due To Loss Of Unsecured Credit [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	10
Additional collateral	4
CECONY [Member] | Additional collateral aggregate fair value down below investment grade [Member]
Derivatives, Fair Value [Line Items]
Collateral posted	1
Additional collateral	$ 43