Derivative Instruments and Hedging Activities - Aggregate Fair Value of Companies' Derivative Instruments with Credit-Risk-Related Contingent Features (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Derivatives, Fair Value [Line Items]
Aggregate fair value – net liabilities	$ 52
Collateral posted	5
Downgrade One Level from Current Ratings
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	1
Downgrade to Below Investment Grade from Current Ratings
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	72
Derivatives in net asset position additional collateral	9
Additional Collateral Required Due To Loss Of Unsecured Credit
Derivatives, Fair Value [Line Items]
Collateral posted	3
CECONY
Derivatives, Fair Value [Line Items]
Aggregate fair value – net liabilities	32
Collateral posted	0
CECONY | Downgrade One Level from Current Ratings
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	0
CECONY | Downgrade to Below Investment Grade from Current Ratings
Derivatives, Fair Value [Line Items]
Derivatives in net liability position additional collateral	$ 43