Derivative Instruments and Hedging Activities - Fair Values of Companies' Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives Liabilities
Fair value of derivative liabilities	$ 124	$ 176
Impact of netting	(74)	(104)
Net derivatives liabilities	50	72
Derivatives Assets [Member]
Derivatives Assets
Fair value of derivative assets	80	86
Impact of netting	(18)	(20)
Net derivatives assets	62	66
Derivatives Assets [Member] | Other current assets [Member]
Derivatives Assets
Fair value of derivative assets	55	64
Derivatives Assets [Member] | Other deferred charges and noncurrent assets [Member]
Derivatives Assets
Fair value of derivative assets	25	22
Derivatives Liabilities [Member] | Fair value of derivative liabilities, Current [Member]
Derivatives Liabilities
Fair value of derivative liabilities	71	122
Derivatives Liabilities [Member] | Fair value of derivative liabilities, Long-term [Member]
Derivatives Liabilities
Fair value of derivative liabilities	53	54
CECONY [Member]
Derivatives Liabilities
Fair value of derivative liabilities	58	83
Impact of netting	(33)	(44)
Net derivatives liabilities	25	39
CECONY [Member] | Derivatives Assets [Member]
Derivatives Assets
Fair value of derivative assets	27	27
Impact of netting	(4)	3
Net derivatives assets	23	30
CECONY [Member] | Derivatives Assets [Member] | Other current assets [Member]
Derivatives Assets
Fair value of derivative assets	18	18
CECONY [Member] | Derivatives Assets [Member] | Other deferred charges and noncurrent assets [Member]
Derivatives Assets
Fair value of derivative assets	9	9
CECONY [Member] | Derivatives Liabilities [Member] | Fair value of derivative liabilities, Current [Member]
Derivatives Liabilities
Fair value of derivative liabilities	40	58
CECONY [Member] | Derivatives Liabilities [Member] | Fair value of derivative liabilities, Long-term [Member]
Derivatives Liabilities
Fair value of derivative liabilities	$ 18	$ 25