Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		$ 6	$ 1	$ 6
Total deferred gains/(losses)	(2)	13	5	(85)
Net deferred gains/(losses)	(2)	19	6	(79)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	4	10	23	(62)
Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		5	1	5
Regulatory liabilities, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		1		1
Deferred derivative losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	11	51	24	89
Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(19)	(60)	(22)	(187)
Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	6	22	3	13
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		6	1	6
Total deferred gains/(losses)	(1)	10	7	(67)
Net deferred gains/(losses)	(1)	16	8	(61)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
CECONY [Member] | Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		5	1	5
CECONY [Member] | Regulatory liabilities, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		1		1
CECONY [Member] | Deferred derivative losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	9	42	19	78
CECONY [Member] | Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(17)	(52)	(19)	(164)
CECONY [Member] | Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	7	20	7	19
Purchased power expense [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	3	9	32	(49)
Purchased power expense [Member] | CECONY [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
Gas purchased for resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(6)		(17)	(2)
Gas purchased for resale [Member] | CECONY [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	7	1	8	(11)
Non-utility revenue [Member] | CECONY [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income