Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory assets
Regulatory assets - long-term	$ 6,552	$ 7,201
Regulatory assets - current	14	29
Total Regulatory Assets	6,566.0	7,230.0
Regulatory liabilities
Regulatory liabilities - noncurrent	1,901	1,728
Regulatory liabilities - current	194	148
Total Regulatory Liabilities	2,095	1,876
Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	572	540
Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	120	133
Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	97	130
Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	85	105
Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	80	98
2014 Rate Plan Base Rate Revenue Deferral [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	118
New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	61
World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	46	62
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	49	50
Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	50
Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	36	40
Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	24	28
Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	21	22
Other Liabilities [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	248	198
Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	118	100
Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	39	14
Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	21	34
Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	16
Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - long-term	2,284	2,730
Future Income Tax [Member]
Regulatory assets
Regulatory assets - long-term	2,191	2,145
Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - long-term	899	938
Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - long-term	348	441
Revenue Taxes [Member]
Regulatory assets
Regulatory assets - long-term	218	207
Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - long-term	144	78
Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	68	83
Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - long-term	59	65
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	83	237
O&R Transition Bond Charges [Member]
Regulatory assets
Regulatory assets - long-term	28	33
Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - long-term	27	28
Property Tax Reconciliation [Member]
Regulatory assets
Regulatory assets - long-term	32	22
Regulatory liabilities
Regulatory liabilities - noncurrent	294	322
Workers' Compensation [Member]
Regulatory assets
Regulatory assets - long-term	10	12
Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - long-term	5	8
Other [Member]
Regulatory assets
Regulatory assets - long-term	156	174
Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - current	1	4
Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	13	25
CECONY [Member]
Regulatory assets
Regulatory assets - long-term	6,011	6,639
Regulatory assets - current	12	26
Total Regulatory Assets	6,023.0	6,665.0
Regulatory liabilities
Regulatory liabilities - noncurrent	1,738	1,598
Regulatory liabilities - current	149	107
Total Regulatory Liabilities	1,887	1,705
CECONY [Member] | Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	475	453
CECONY [Member] | Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	120	133
CECONY [Member] | Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	97	130
CECONY [Member] | Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	85	105
CECONY [Member] | Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	64	87
CECONY [Member] | 2014 Rate Plan Base Rate Revenue Deferral [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	118
CECONY [Member] | New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	57
CECONY [Member] | World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	46	62
CECONY [Member] | Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	41	50
CECONY [Member] | Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	42
CECONY [Member] | Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	36	40
CECONY [Member] | Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	22	20
CECONY [Member] | Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	20	18
CECONY [Member] | Other Liabilities [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	221	178
CECONY [Member] | Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	82	66
CECONY [Member] | Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	31	11
CECONY [Member] | Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	20	30
CECONY [Member] | Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	16
CECONY [Member] | Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - long-term	2,185	2,610
CECONY [Member] | Future Income Tax [Member]
Regulatory assets
Regulatory assets - long-term	2,078	2,030
CECONY [Member] | Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - long-term	795	830
CECONY [Member] | Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - long-term	252	334
CECONY [Member] | Revenue Taxes [Member]
Regulatory assets
Regulatory assets - long-term	207	196
CECONY [Member] | Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - long-term	134	74
CECONY [Member] | Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	68	83
CECONY [Member] | Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - long-term	57	62
CECONY [Member] | Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	57	211
CECONY [Member] | Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - long-term	27	28
CECONY [Member] | Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	294	322
CECONY [Member] | Workers' Compensation [Member]
Regulatory assets
Regulatory assets - long-term	10	12
CECONY [Member] | Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - long-term	5	7
CECONY [Member] | Other [Member]
Regulatory assets
Regulatory assets - long-term	136	162
CECONY [Member] | Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - current		4
CECONY [Member] | Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	$ 12	$ 22