Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory assets - long-term	$ 8,897	$ 9,337
Regulatory assets - current	77	164
Regulatory liabilities - long-term	1,102	977
Regulatory liabilities - current	231	118
Con Edison [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	8,897	9,337
Regulatory assets - current	77	164
Regulatory liabilities - long-term	1,102	977
Total Regulatory Assets	8,974	9,501
Regulatory liabilities - current	231	118
Total Regulatory Liabilities	1,333	1,095
Con Edison [Member] | Unrecognized Pension And Other Postretirement Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	5,211	5,852
Con Edison [Member] | Future Income Tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1,888	1,798
Con Edison [Member] | Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	722	681
Con Edison [Member] | Pension And Other Post Retirement Benefits Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	205	198
Con Edison [Member] | Revenue Taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	177	163
Con Edison [Member] | Surcharge For New York State Assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	135	90
Con Edison [Member] | Deferred Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	120	128
Con Edison [Member] | Net Electric Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	107	121
Con Edison [Member] | Deferred Derivative Losses - Long-Term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	47	60
Regulatory assets - current	75	164
Con Edison [Member] | O&R Transition Bond Charges [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	40	44
Con Edison [Member] | Preferred Stock Redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	29
Con Edison [Member] | Workers' Compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	19	23
Con Edison [Member] | Property Tax Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	15	13
Regulatory liabilities - long-term	137	35
Con Edison [Member] | Recoverable Energy Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1	14
Regulatory assets - current	2
Con Edison [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	181	152
Con Edison [Member] | Allowance For Cost Of Removal Less Salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	488	448
Con Edison [Member] | Net Unbilled Revenue Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	121	104
Con Edison [Member] | World Trade Center Settlement Proceeds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	62	62
Con Edison [Member] | Long-Term Interest Rate Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	54	30
Con Edison [Member] | Carrying Charges On Transmission And Distribution Net Plant [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	36	38
Con Edison [Member] | Expenditure Prudence Proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	15	11
Con Edison [Member] | Gas Line Losses [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	14	21
Con Edison [Member] | Energy Efficiency Programs [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	5	22
Con Edison [Member] | Other Liabilities [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	170	206
Con Edison [Member] | Revenue Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	107	66
Con Edison [Member] | Electric Surcharge Offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	60
Con Edison [Member] | Refundable Energy Costs - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	58	51
Con Edison [Member] | Deferred Derivative Gains, Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	6	1
CECONY [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	8,285	8,661
Regulatory assets - current	62	140
Regulatory liabilities - long-term	978	861
Total Regulatory Assets	8,347	8,801
Regulatory liabilities - current	199	79
Total Regulatory Liabilities	1,177	940
CECONY [Member] | Unrecognized Pension And Other Postretirement Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	4,972	5,554
CECONY [Member] | Future Income Tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1,810	1,724
CECONY [Member] | Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	607	564
CECONY [Member] | Pension And Other Post Retirement Benefits Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	174	157
CECONY [Member] | Revenue Taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	171	158
CECONY [Member] | Surcharge For New York State Assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	125	82
CECONY [Member] | Deferred Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	78	80
CECONY [Member] | Net Electric Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	107	121
CECONY [Member] | Deferred Derivative Losses - Long-Term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	25	44
Regulatory assets - current	62	140
CECONY [Member] | Preferred Stock Redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	29
CECONY [Member] | Workers' Compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	19	23
CECONY [Member] | Property Tax Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	137	35
CECONY [Member] | Recoverable Energy Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1	14
CECONY [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	167	140
CECONY [Member] | Allowance For Cost Of Removal Less Salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	407	372
CECONY [Member] | Net Unbilled Revenue Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	121	104
CECONY [Member] | World Trade Center Settlement Proceeds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	62	62
CECONY [Member] | Long-Term Interest Rate Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	54	30
CECONY [Member] | Carrying Charges On Transmission And Distribution Net Plant [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	16	14
CECONY [Member] | Expenditure Prudence Proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	15	11
CECONY [Member] | Gas Line Losses [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	14	21
CECONY [Member] | Energy Efficiency Programs [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	5	20
CECONY [Member] | Other Liabilities [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	147	192
CECONY [Member] | Revenue Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	103	66
CECONY [Member] | Electric Surcharge Offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	60
CECONY [Member] | Refundable Energy Costs - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	30	12
CECONY [Member] | Deferred Derivative Gains, Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	$ 6	$ 1