Derivative Instruments and Hedging Activities - Realized and Unrealized Gains or Losses on Commodity Derivatives (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ (1)	$ (11)	$ 0	$ 26
Total deferred gains/(losses)	(31)	(36)	(59)	67
Net deferred gains/(losses)	(32)	(47)	(59)	93
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(52)	(37)	(134)	69
Deferred Derivative Gains, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	(6)	0	25
Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	(5)	0	1
Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	8	(6)	40	10
Recoverable energy costs
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(53)	(33)	(92)	54
Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	14	3	(7)	3
CECONY
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	(10)	1	21
Total deferred gains/(losses)	(28)	(31)	(52)	53
Net deferred gains/(losses)	(29)	(41)	(51)	74
CECONY | Deferred Derivative Gains, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(1)	(5)	1	20
CECONY | Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	(5)	0	1
CECONY | Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	8	(5)	40	10
CECONY | Recoverable energy costs
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(49)	(29)	(87)	41
CECONY | Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	13	3	(5)	2
Non Utility Operating Revenue
Pre-tax gain/(loss) recognized in income
Unrealized gain (loss) on derivatives			(3)
Purchased power expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(31)	(28)	(60)	134
Unrealized gain (loss) on derivatives	12	(1)	6	14
Gas purchased for resale
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(26)	(29)	(94)	(75)
Non-utility revenue
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	$ 5	$ 20	$ 20	$ 10