Derivative Instruments and Hedging Activities - Fair Values of Companies' Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative Assets
Fair value of derivative assets	$ 240	$ 166
Impact of netting	(130)	(84)
Net derivatives assets	110	82
Derivative Liabilities
Fair value of derivative liabilities	124	113
Impact of netting	(114)	(98)
Net derivatives liabilities	10	15
Other Current Assets [Member]
Derivative Assets
Fair value of derivative assets	192	134
Other Deferred Charges and Noncurrent Assets [Member]
Derivative Assets
Fair value of derivative assets	48	32
Fair Value of Derivative Liabilities, Current [Member]
Derivative Liabilities
Fair value of derivative liabilities	82	82
Fair Value of Derivative Liabilities, Noncurrent [Member]
Derivative Liabilities
Fair value of derivative liabilities	42	31
CECONY [Member]
Derivative Assets
Fair value of derivative assets	103	41
Impact of netting	(55)	(16)
Net derivatives assets	48	25
Derivative Liabilities
Fair value of derivative liabilities	69	51
Impact of netting	(65)	(37)
Net derivatives liabilities	4	14
CECONY [Member] \| Other Current Assets [Member]
Derivative Assets
Fair value of derivative assets	75	27
CECONY [Member] \| Other Deferred Charges and Noncurrent Assets [Member]
Derivative Assets
Fair value of derivative assets	28	14
CECONY [Member] \| Fair Value of Derivative Liabilities, Current [Member]
Derivative Liabilities
Fair value of derivative liabilities	40	32
CECONY [Member] \| Fair Value of Derivative Liabilities, Noncurrent [Member]
Derivative Liabilities
Fair value of derivative liabilities	$ 29	$ 19

Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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