Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended				9 Months Ended		12 Months Ended				9 Months Ended					9 Months Ended					9 Months Ended						9 Months Ended										9 Months Ended													9 Months Ended				9 Months Ended												12 Months Ended				9 Months Ended		12 Months Ended				9 Months Ended					9 Months Ended					9 Months Ended				9 Months Ended						9 Months Ended		12 Months Ended			9 Months Ended		12 Months Ended			9 Months Ended					9 Months Ended					9 Months Ended						9 Months Ended										9 Months Ended
	Dec. 31, 2011 Con Edison [Member]		Sep. 30, 2012 Con Edison [Member]		Sep. 30, 2012 Con Edison [Member] Derivatives Assets [Member]		Dec. 31, 2011 CECONY [Member]		Sep. 30, 2012 CECONY [Member]		Sep. 30, 2012 CECONY [Member] Derivatives Assets [Member]		Sep. 30, 2012 Commodity [Member] Con Edison [Member]	Dec. 31, 2011 Commodity [Member] Con Edison [Member]		Sep. 30, 2012 Commodity [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Commodity [Member] CECONY [Member]	Dec. 31, 2011 Commodity [Member] CECONY [Member]		Sep. 30, 2012 Commodity [Member] CECONY [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Other Assets [Member] Con Edison [Member]		Dec. 31, 2011 Other Assets [Member] Con Edison [Member]		Sep. 30, 2012 Other Assets [Member] Con Edison [Member] Derivatives Assets [Member]		Sep. 30, 2012 Other Assets [Member] CECONY [Member]		Dec. 31, 2011 Other Assets [Member] CECONY [Member]		Sep. 30, 2012 Other Assets [Member] CECONY [Member] Derivatives Assets [Member]	Sep. 30, 2012 Interest Rate Contract [Member] Con Edison [Member]	Dec. 31, 2011 Interest Rate Contract [Member] Con Edison [Member]		Sep. 30, 2012 Interest Rate Contract [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 1 [Member] Con Edison [Member]		Dec. 31, 2011 Level 1 [Member] Con Edison [Member]		Sep. 30, 2012 Level 1 [Member] CECONY [Member]		Dec. 31, 2011 Level 1 [Member] CECONY [Member]		Sep. 30, 2012 Level 1 [Member] Commodity [Member] Con Edison [Member]	Dec. 31, 2011 Level 1 [Member] Commodity [Member] Con Edison [Member]		Sep. 30, 2012 Level 1 [Member] Commodity [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 1 [Member] Commodity [Member] CECONY [Member]	Dec. 31, 2011 Level 1 [Member] Commodity [Member] CECONY [Member]	Sep. 30, 2012 Level 1 [Member] Commodity [Member] CECONY [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 1 [Member] Other Assets [Member] Con Edison [Member]		Dec. 31, 2011 Level 1 [Member] Other Assets [Member] Con Edison [Member]		Sep. 30, 2012 Level 1 [Member] Other Assets [Member] Con Edison [Member] Derivatives Assets [Member]	Sep. 30, 2012 Level 1 [Member] Other Assets [Member] CECONY [Member]		Dec. 31, 2011 Level 1 [Member] Other Assets [Member] CECONY [Member]		Sep. 30, 2012 Level 1 [Member] Other Assets [Member] CECONY [Member] Derivatives Assets [Member]	Dec. 31, 2011 Level 2 [Member] Con Edison [Member]		Sep. 30, 2012 Level 2 [Member] Con Edison [Member]		Sep. 30, 2012 Level 2 [Member] Con Edison [Member] Derivatives Assets [Member]		Dec. 31, 2011 Level 2 [Member] CECONY [Member]		Sep. 30, 2012 Level 2 [Member] CECONY [Member]		Sep. 30, 2012 Level 2 [Member] CECONY [Member] Derivatives Assets [Member]		Sep. 30, 2012 Level 2 [Member] Commodity [Member] Con Edison [Member]	Dec. 31, 2011 Level 2 [Member] Commodity [Member] Con Edison [Member]		Sep. 30, 2012 Level 2 [Member] Commodity [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 2 [Member] Commodity [Member] CECONY [Member]	Dec. 31, 2011 Level 2 [Member] Commodity [Member] CECONY [Member]		Sep. 30, 2012 Level 2 [Member] Commodity [Member] CECONY [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 2 [Member] Other Assets [Member] Con Edison [Member]		Sep. 30, 2012 Level 2 [Member] Other Assets [Member] Con Edison [Member] Derivatives Assets [Member]		Sep. 30, 2012 Level 2 [Member] Other Assets [Member] CECONY [Member]		Sep. 30, 2012 Level 2 [Member] Other Assets [Member] CECONY [Member] Derivatives Assets [Member]	Sep. 30, 2012 Level 2 [Member] Interest Rate Contract [Member] Con Edison [Member]	Sep. 30, 2012 Level 2 [Member] Interest Rate Contract [Member] Con Edison [Member] Derivatives Liabilities [Member]		Dec. 31, 2011 Level 3 [Member] Con Edison [Member]		Sep. 30, 2012 Level 3 [Member] Con Edison [Member]	Sep. 30, 2012 Level 3 [Member] Con Edison [Member] Derivatives Assets [Member]		Dec. 31, 2011 Level 3 [Member] CECONY [Member]		Sep. 30, 2012 Level 3 [Member] CECONY [Member]	Sep. 30, 2012 Level 3 [Member] CECONY [Member] Derivatives Assets [Member]		Sep. 30, 2012 Level 3 [Member] Commodity [Member] Con Edison [Member]	Dec. 31, 2011 Level 3 [Member] Commodity [Member] Con Edison [Member]		Sep. 30, 2012 Level 3 [Member] Commodity [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 3 [Member] Commodity [Member] CECONY [Member]	Dec. 31, 2011 Level 3 [Member] Commodity [Member] CECONY [Member]		Sep. 30, 2012 Level 3 [Member] Commodity [Member] CECONY [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Level 3 [Member] Other Assets [Member] Con Edison [Member]		Dec. 31, 2011 Level 3 [Member] Other Assets [Member] Con Edison [Member]		Sep. 30, 2012 Level 3 [Member] Other Assets [Member] Con Edison [Member] Derivatives Assets [Member]		Sep. 30, 2012 Level 3 [Member] Other Assets [Member] CECONY [Member]		Dec. 31, 2011 Level 3 [Member] Other Assets [Member] CECONY [Member]		Sep. 30, 2012 Level 3 [Member] Other Assets [Member] CECONY [Member] Derivatives Assets [Member]	Sep. 30, 2012 Level 3 [Member] Interest Rate Contract [Member] Con Edison [Member]	Dec. 31, 2011 Level 3 [Member] Interest Rate Contract [Member] Con Edison [Member]		Sep. 30, 2012 Level 3 [Member] Interest Rate Contract [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Netting Adjustments [Member] Con Edison [Member]		Dec. 31, 2011 Netting Adjustments [Member] Con Edison [Member]		Sep. 30, 2012 Netting Adjustments [Member] CECONY [Member]		Dec. 31, 2011 Netting Adjustments [Member] CECONY [Member]		Sep. 30, 2012 Netting Adjustments [Member] Commodity [Member] Con Edison [Member]		Dec. 31, 2011 Netting Adjustments [Member] Commodity [Member] Con Edison [Member]		Sep. 30, 2012 Netting Adjustments [Member] Commodity [Member] Con Edison [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Netting Adjustments [Member] Commodity [Member] CECONY [Member]		Dec. 31, 2011 Netting Adjustments [Member] Commodity [Member] CECONY [Member]		Sep. 30, 2012 Netting Adjustments [Member] Commodity [Member] CECONY [Member] Derivatives Liabilities [Member]		Sep. 30, 2012 Netting Adjustments [Member] Other Assets [Member] Con Edison [Member]		Sep. 30, 2012 Netting Adjustments [Member] Other Assets [Member] Con Edison [Member] Derivatives Assets [Member]		Sep. 30, 2012 Netting Adjustments [Member] Other Assets [Member] CECONY [Member]		Sep. 30, 2012 Netting Adjustments [Member] Other Assets [Member] CECONY [Member] Derivatives Assets [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Transfer in	$ 32	[1],[2],[3]			$ 105	[1],[3]	$ 31	[1],[2],[3]			$ 95	[1],[3]				$ 60	[1],[3]				$ 22	[1],[3]					$ 24	[1],[3]									$ 8	[1],[3]												$ 7	[1],[3]			$ 7	[1],[3]											$ 26	[1],[2],[3]			$ 105	[1],[3]	$ 25	[1],[2],[3]			$ 95	[1],[3]				$ 50	[1],[3]				$ 12	[1],[3]			$ 24	[1],[3]					$ 8	[1],[3]	$ 6	[1],[2],[3]				$ 6	[1],[2],[3]							$ 3	[1],[3]				$ 3	[1],[3]
Transfer out	(32)	[1],[2],[3]			(105)	[1],[3]	(31)	[1],[2],[3]			(95)	[1],[3]				(60)	[1],[3]				(22)	[1],[3]					(24)	[1],[3]									(8)	[1],[3]												(2)	[1],[3]															(6)	[1],[2],[3]					(6)	[1],[2],[3]								(10)	[1],[3]				(10)	[1],[3]											(26)	[1],[2],[3]		(105)	[1],[3]	(25)	[1],[2],[3]		(95)	[1],[3]				(48)	[1],[3]				(12)	[1],[3]					(24)	[1],[3]									(8)	[1],[3]
Commodity Total	245		259				190		219																		51						27						105		79		98		76															2					2	64		177				8		112																72				16				186		23			101		12																	23						12						(46)	[4]	(84)	[4]	(3)	[4]	5	[4]															(46)	[4]			(3)	[4]
Derivative assets			208	[5]					192	[5]				70	[6]				24	[6]			51	[6]	175	[5]	208		27	[6]	166	[5]	192						103	[5]			96	[5]				3	[6]							2	[6]	76	[5]	103	2	[6]	76	[5]	96			105	[5]					96	[5]				64	[6]				8	[6]			48	[6]			16	[6]	1															87	[6]				11	[6]			47	[6]	99	[5]	105		12	[6]	90	[5]	95																(84)	[4],[6]					5	[4],[6]			(46)	[4],[6]			(3)	[4],[6]
Total liabilities	217		90				122		42																														9		12		8		4																					242		160				141		70																								157		26			18																															(105)	[4]	(194)	[4]	(36)	[4]	(41)	[4]
Derivative liabilities	$ 209	[6]					$ 122	[6]					$ 84	$ 209		$ 84	[6]	$ 42	$ 122		$ 42	[6]												$ 6	$ 8	[7]	$ 6	[7]			$ 12	[6]			$ 4	[6]	$ 4	$ 12		$ 9	[6]	$ 1	$ 4	$ 8	[6]											$ 242	[6]					$ 141	[6]					$ 114	$ 222		$ 154	[6]	$ 68	$ 122		$ 70	[6]								$ (2)	$ 6	[7]	$ 149	[6]				$ 18	[6]				$ 71	$ 169		$ 26	[6]	$ 9	$ 37															$ 8	$ 8	[7]					$ (194)	[4],[6]			$ (41)	[4],[6]	$ (105)	[4]	$ (194)	[4]	$ (105)	[4],[6]	$ (36)	[4]	$ (41)	[4]	$ (36)	[4],[6]

[1] The Companies' policy is to recognize transfers into and transfers out of the levels at the end of the reporting period.

[2] Transferred from Level 3 to Level 2 because of availability of observable market data due to decrease in the terms of certain contracts from beyond one year as of December 31, 2010 to less than one year as of December 31, 2011.

[3] Transferred between Level 3 and Levels 1 and 2 because of reassessment of the levels in the fair value hierarchy within which certain inputs fall. Other assets and interest rate contract were transferred as of March 31, 2012.

[4] Amounts represent the impact of legally-enforceable master netting agreements that allow the Companies to net gain and loss positions and cash collateral held or placed with the same counterparties.

[5] Other assets are comprised of assets such as life insurance contracts within the deferred compensation plan and non-qualified retirement plans.

[6] A portion of the commodity derivatives categorized in Level 3 is valued using an internally developed model with observable inputs. The models also include some less readily observable inputs resulting in the classification of the entire contract as Level 3. See Note J.

[7] See Note J.