Derivative Instruments and Hedging Activities - Realized and Unrealized Gains or Losses on Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ (9)	$ 14
Total deferred gains/(losses)	(56)	35
Net deferred gains/(losses)	(65)	49
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(123)	72
Deferred Derivative Gains - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(10)	14
Deferred Derivative Gains, Long-term [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	1
Deferred Derivative Losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(75)	47
Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	36	(39)
Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(17)	27
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(6)	11
Total deferred gains/(losses)	(61)	14
Net deferred gains/(losses)	(67)	25
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)	11
CECONY [Member] | Deferred Derivative Gains, 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)	(70)	38
CECONY [Member] | Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	26	(37)
CECONY [Member] | Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(17)	13
Purchased Power Costs [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(37)	90
Gas Purchased for Resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(115)	(27)
Non-utility Revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	$ 29	$ 9