Derivative Instruments and Hedging Activities - Fair Values of Companies' Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative Liabilities
Fair value of derivative liabilities	$ 95	$ 113
Impact of netting	(89)	(98)
Net derivatives liabilities	6	15
Derivatives Assets [Member]
Derivative Assets
Fair value of derivative assets	232	166
Impact of netting	(69)	(84)
Net derivatives assets	163	82
Derivatives Assets [Member] | Other Current Assets [Member]
Derivative Assets
Fair value of derivative assets	193	134
Derivatives Assets [Member] | Other Deferred Charges and Noncurrent Assets [Member]
Derivative Assets
Fair value of derivative assets	39	32
Derivatives Liabilities [Member] | Fair Value of Derivative Liabilities, Current [Member]
Derivative Liabilities
Fair value of derivative liabilities	68	82
Derivatives Liabilities [Member] | Fair Value of Derivative Liabilities, Long-term [Member]
Derivative Liabilities
Fair value of derivative liabilities	27	31
CECONY [Member]
Derivative Liabilities
Fair value of derivative liabilities	36	51
Impact of netting	(33)	(37)
Net derivatives liabilities	3	14
CECONY [Member] | Derivatives Assets [Member]
Derivative Assets
Fair value of derivative assets	83	41
Impact of netting	(20)	(16)
Net derivatives assets	63	25
CECONY [Member] | Derivatives Assets [Member] | Other Current Assets [Member]
Derivative Assets
Fair value of derivative assets	65	27
CECONY [Member] | Derivatives Assets [Member] | Other Deferred Charges and Noncurrent Assets [Member]
Derivative Assets
Fair value of derivative assets	18	14
CECONY [Member] | Derivatives Liabilities [Member] | Fair Value of Derivative Liabilities, Current [Member]
Derivative Liabilities
Fair value of derivative liabilities	21	32
CECONY [Member] | Derivatives Liabilities [Member] | Fair Value of Derivative Liabilities, Long-term [Member]
Derivative Liabilities
Fair value of derivative liabilities	$ 15	$ 19