Derivative Instruments and Hedging Activities - Realized and Unrealized Gains or Losses on Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 3	$ 34
Total deferred gains/(losses)	24	113
Net deferred gains/(losses)	27	147
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	21	137
Deferred Derivative Gains - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	3	30
Deferred Derivative Gains, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		4
Deferred Derivative Losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	43	17
Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		94
Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(19)	2
CECONY [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	3	29
Total deferred gains/(losses)	23	96
Net deferred gains/(losses)	26	125
CECONY [Member] | Deferred Derivative Gains - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	3	25
CECONY [Member] | Deferred Derivative Gains, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)		4
CECONY [Member] | Deferred Derivative Losses, Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	41	17
CECONY [Member] | Recoverable Energy Costs - Current [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(2)	77
CECONY [Member] | Deferred Derivative Losses, Noncurrent [Member]
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(16)	2
Purchased Power Costs [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	21	175
Gas Purchased for Resale [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	(42)	(14)
Non-utility Revenue [Member]
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	$ 42	$ (24)