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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 3	$ (9)	$ 38	$ 2
Total deferred gains/(losses)	(12)	(48)	102	8
Net deferred gains/(losses)	(9)	(57)	140	10
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(31)	(42)	105	20
Deferred Derivative Gains - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	1	(7)	31	2
Regulatory Liabilities, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		(2)	7
Deferred Derivative Losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(2)	(24)	15	14
Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(7)	(14)	87	(3)
Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(3)	(10)		(3)
Deferred Derivative Gains, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	2
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	3	(8)	31	1
Total deferred gains/(losses)	(11)	(41)	84	7
Net deferred gains/(losses)	(8)	(49)	115	8
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	(7)	25	1
CECONY [Member] | Regulatory Liabilities, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		(1)	6
CECONY [Member] | Deferred Derivative Losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(2)	(23)	15	9
CECONY [Member] | Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(6)	(12)	70	(2)
CECONY [Member] | Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(3)	(6)	(1)
CECONY [Member] | Deferred Derivative Gains, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	2
Purchased Power Costs [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(13)	(37)	161	30
Gas Purchased for Resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(32)	(7)	(46)	(11)
Non-utility Revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	$ 14	$ 2	$ (10)	$ 1