Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ (9)	$ (1)	$ 2
Total deferred gains/(losses)	(48)	11	8	(100)
Net deferred gains/(losses)	(57)	10	10	(100)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(42)	18	20	(72)
Deferred derivative gains, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(7)	(1)	2
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)	(24)	66	14	38
Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(14)	(63)	(3)	(127)
Regulatory assets, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(10)	8	(3)	(11)
Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	2	(8)	1	(11)
Purchased power expense [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(37)	27	30	(59)
Gas purchased for resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(7)	(1)	(11)	(2)
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(8)	(1)	1
Total deferred gains/(losses)	(41)	15	7	(77)
Net deferred gains/(losses)	(49)	14	8	(77)
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)	(7)	(1)	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)	(23)	55	9	36
CECONY [Member] | Recoverable energy costs, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(12)	(56)	(2)	(112)
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)	16		(1)
CECONY [Member] | Non-utility revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income
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