Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 11	$ 1
Total deferred gains/(losses)	56	(120)
Net deferred gains/(losses)	67	(119)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	62	(90)
Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	9	1
Regulatory liabilities, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	2
Deferred derivative losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	38	(28)
Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	11	(74)
Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	7	(18)
Purchased power expense [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	67	(86)
Gas purchased for resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(4)	(1)
Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(1)	(3)
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	9	1
Total deferred gains/(losses)	48	(92)
Net deferred gains/(losses)	57	(91)
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)	8	1
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)	32	(19)
CECONY [Member] | Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	10	(56)
CECONY [Member] | Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	6	(17)
CECONY [Member] | Purchased power expense [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
CECONY [Member] | Gas purchased for resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
CECONY [Member] | Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income