Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	$ (1)	$ (4)
Total deferred gains/ (losses)	(108)	(210)
Net deferred gains/ (losses)	(109)	(214)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(70)	(126)
Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	(1)
Total deferred gains/ (losses)		(3)
Regulatory liabilities, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)		(1)
Deferred derivative losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	95	26
Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	(220)	(247)
Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	17	11
Purchased power expense [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(54)	(78)
Gas purchased for resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(5)	(18)
Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(11)	(30)
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	(1)	(3)
Total deferred gains/ (losses)	(88)	(170)
Net deferred gains/ (losses)	(89)	(173)
CECONY [Member] | Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	(1)
Total deferred gains/ (losses)		(2)
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)
CECONY [Member] | Deferred derivative losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	80	11
CECONY [Member] | Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	(192)	(185)
CECONY [Member] | Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/ (losses)	$ 24	$ 4