Derivative Instruments And Hedging Activities - Changes In Fair Values Of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Con Edison [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	$ 6	[1]	$ (1)	[1]	$ 6	[1]	$ 3	[1]
Total deferred gains/(losses)	13	[1]	(53)	[1]	(85)	[1]	(59)	[1]
Net deferred gains/(losses)	19	[1]	(54)	[1]	(79)	[1]	(56)	[1]
Total pre-tax gain/(loss) recognized in income	10	[1]	40	[1]	(62)	[1]	120	[1]
Con Edison [Member] | Deferred Derivative Gains, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	5	[1]			5	[1]
Con Edison [Member] | Deferred Derivative Gains, Long-term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	1	[1]	(2)	[1]	1	[1]
Total deferred gains/(losses)								[1]
Con Edison [Member] | Deferred Derivative Losses, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	51	[1]	12	[1]	89	[1]	80	[1]
Con Edison [Member] | Recoverable Energy Costs, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	(60)	[1]	(75)	[1]	(187)	[1]	(177)	[1]
Con Edison [Member] | Regulatory Assets, Long-Term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	22	[1]	10	[1]	13	[1]	38	[1]
Con Edison [Member] | Regulatory Liabilities, Long-Term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)			1	[1]			3	[1]
CECONY [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	6	[1]			6	[1]	3	[1]
Total deferred gains/(losses)	10	[1]	(40)	[1]	(67)	[1]	(47)	[1]
Net deferred gains/(losses)	16	[1]	(40)	[1]	(61)	[1]	(44)	[1]
CECONY [Member] | Deferred Derivative Gains, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	5	[1]			5	[1]
CECONY [Member] | Deferred Derivative Gains, Long-term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	1	[1]	(1)	[1]	1	[1]
Total deferred gains/(losses)								[1]
CECONY [Member] | Deferred Derivative Losses, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	42	[1]	7	[1]	78	[1]	60	[1]
CECONY [Member] | Recoverable Energy Costs, Current [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	(52)	[1]	(53)	[1]	(164)	[1]	(134)	[1]
CECONY [Member] | Regulatory Assets, Long-Term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)	20	[1]	6	[1]	19	[1]	27	[1]
CECONY [Member] | Regulatory Liabilities, Long-Term [Member]
Derivatives, Fair Value [Line Items]
Total deferred gains/(losses)			1	[1]			3	[1]
Non-Utility Revenue [Member]
Derivatives, Fair Value [Line Items]
Total pre-tax gain/(loss) recognized in income			(10)		(13)		(35)
Non-Utility Revenue [Member] | Con Edison [Member]
Derivatives, Fair Value [Line Items]
Pre-tax gain/(loss) recognized in income	1	[1],[2]	5	[1],[3]	(11)	[1],[4]	22	[1],[5]
Purchased Power Expense [Member]
Derivatives, Fair Value [Line Items]
Total pre-tax gain/(loss) recognized in income	30		(1)		75		59
Purchased Power Expense [Member] | Con Edison [Member]
Derivatives, Fair Value [Line Items]
Pre-tax gain/(loss) recognized in income	9	[1],[2]	29	[1],[3]	(49)	[1],[4]	81	[1],[5]
Gas Purchased For Resale [Member] | Con Edison [Member]
Derivatives, Fair Value [Line Items]
Pre-tax gain/(loss) recognized in income			$ 6	[1]	$ (2)	[1]	$ 17	[1]

[1] Qualifying derivative contracts, which have been designated as normal purchases or normal sales contracts, are not reported at fair value under the accounting rules for derivatives and hedging and, therefore, are excluded from the table.

[2] For the three months ended September 30, 2012, Con Edison recorded in purchased power expense an unrealized pre-tax gain/(loss) of $30 million.

[3] For the three months ended September 30, 2011, Con Edison recorded in non-utility revenues and purchased power expense an unrealized pre-tax gain/(loss) of $(10) million and $(1) million, respectively.

[4] For the nine months ended September 30, 2012, Con Edison recorded in non-utility revenues and purchased power expense an unrealized pre-tax gain/(loss) of $(13) million and $75 million, respectively.

[5] For the nine months ended September 30, 2011, Con Edison recorded in non-utility revenues and purchased power expense an unrealized pre-tax gain/(loss) of $(35) million and $59 million, respectively.