Derivative Instruments and Hedging Activities - Fair Values of Companies' Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives assets [Member]
Derivatives Assets
Fair value of derivative assets	$ 78	$ 86
Impact of netting	(18)	(20)
Net derivatives assets	60	66
Derivatives assets [Member] | Other current assets [Member]
Derivatives Assets
Fair value of derivative assets	56	64
Derivatives assets [Member] | Other deferred charges and noncurrent assets [Member]
Derivatives Assets
Fair value of derivative assets	22	22
Derivatives liabilities [Member]
Derivatives Liabilities
Fair value of derivative liabilities	146	176
Impact of netting	(87)	(104)
Net derivatives liabilities	59	72
Derivatives liabilities [Member] | Fair value of derivative liabilities, Current [Member]
Derivatives Liabilities
Fair value of derivative liabilities	91	122
Derivatives liabilities [Member] | Fair value of derivative liabilities, Long-term [Member]
Derivatives Liabilities
Fair value of derivative liabilities	55	54
CECONY [Member] | Derivatives assets [Member]
Derivatives Assets
Fair value of derivative assets	22	27
Impact of netting	2	3
Net derivatives assets	24	30
CECONY [Member] | Derivatives assets [Member] | Other current assets [Member]
Derivatives Assets
Fair value of derivative assets	15	18
CECONY [Member] | Derivatives assets [Member] | Other deferred charges and noncurrent assets [Member]
Derivatives Assets
Fair value of derivative assets	7	9
CECONY [Member] | Derivatives liabilities [Member]
Derivatives Liabilities
Fair value of derivative liabilities	71	83
Impact of netting	(37)	(44)
Net derivatives liabilities	34	39
CECONY [Member] | Derivatives liabilities [Member] | Fair value of derivative liabilities, Current [Member]
Derivatives Liabilities
Fair value of derivative liabilities	48	58
CECONY [Member] | Derivatives liabilities [Member] | Fair value of derivative liabilities, Long-term [Member]
Derivatives Liabilities
Fair value of derivative liabilities	$ 23	$ 25