Derivative Instruments and Hedging Activities - Realized and Unrealized Gains or Losses on Commodity Derivatives (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ (2)	$ 3	$ 1	$ 38
Total deferred gains/(losses)	(53)	(12)	(28)	102
Net deferred gains/(losses)	(55)	(9)	(27)	140
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(103)	(31)	(82)	105
Deferred derivative gains
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(2)	1	1	31
Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	2	0	7
Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(11)	(2)	32	15
Recoverable energy costs
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(40)	(7)	(39)	87
Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(2)	(3)	(21)	0
CECONY
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	3	2	31
Total deferred gains/(losses)	(47)	(11)	(24)	84
Net deferred gains/(losses)	(48)	(8)	(22)	115
CECONY | Deferred derivative gains
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	1	2	25
CECONY | Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	2	0	6
CECONY | Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(10)	(2)	32	15
CECONY | Recoverable energy costs
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(36)	(6)	(38)	70
CECONY | Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	(3)	(18)	(1)
Non Utility Operating Revenue [Member]
Pre-tax gain/(loss) recognized in income
Unrealized gain (loss) on derivatives	1		(3)
Purchased power expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(50)	(13)	(28)	161
Unrealized gain (loss) on derivatives	(17)	(5)	(5)	15
Gas purchased for resale
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(26)	(32)	(69)	(46)
Non-utility revenue
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	$ (27)	$ 14	$ 15	$ (10)