Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Regulatory assets
Regulatory assets - long-term	$ 6,782	$ 7,201
Regulatory assets - current	24	29
Total Regulatory Assets	6,806.0	7,230.0
Regulatory liabilities
Regulatory liabilities - noncurrent	1,861	1,728
Regulatory liabilities - current	227	148
Total Regulatory Liabilities	2,088	1,876
Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	569	540
Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	115	133
Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	108	130
Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	92	105
Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	87	98
2014 Rate Plan Base Rate Revenue Deferral [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	82
New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	69
World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	52	62
Other Postretirement Benefits Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	49	50
Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	41
Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	37	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 [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	233	198
Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	147	100
Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	45	14
Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	18	34
Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	17
Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - long-term	2,438	2,730
Future Income Tax [Member]
Regulatory assets
Regulatory assets - long-term	2,158	2,145
Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - long-term	911	938
Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - long-term	382	441
Revenue Taxes [Member]
Regulatory assets
Regulatory assets - long-term	214	207
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	135	237
Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - long-term	110	78
Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	73	83
Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - long-term	61	65
O&R Transition Bond Charges [Member]
Regulatory assets
Regulatory assets - long-term	30	33
Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - long-term	28	28
Property Tax Reconciliation [Member]
Regulatory assets
Regulatory assets - long-term	28	22
Regulatory liabilities
Regulatory liabilities - noncurrent	282	322
Workers' Compensation [Member]
Regulatory assets
Regulatory assets - long-term	12	12
Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - long-term	8	8
Other [Member]
Regulatory assets
Regulatory assets - long-term	194	174
Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - current	16	4
Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	8	25
CECONY [Member]
Regulatory assets
Regulatory assets - long-term	6,235	6,639
Regulatory assets - current	21	26
Total Regulatory Assets	6,256.0	6,665.0
Regulatory liabilities
Regulatory liabilities - noncurrent	1,702	1,598
Regulatory liabilities - current	180	107
Total Regulatory Liabilities	1,882	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	115	133
CECONY [Member] | Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	108	130
CECONY [Member] | Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	92	105
CECONY [Member] | Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	72	87
CECONY [Member] | 2014 Rate Plan Base Rate Revenue Deferral [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	82
CECONY [Member] | New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	65
CECONY [Member] | World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	52	62
CECONY [Member] | Other Postretirement Benefits Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	43	50
CECONY [Member] | Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	32
CECONY [Member] | Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	37	40
CECONY [Member] | Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	21	20
CECONY [Member] | Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	20	18
CECONY [Member] | Other [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	206	178
CECONY [Member] | Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	110	66
CECONY [Member] | Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	36	11
CECONY [Member] | Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	18	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	2,331	2,610
CECONY [Member] | Future Income Tax [Member]
Regulatory assets
Regulatory assets - long-term	2,049	2,030
CECONY [Member] | Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - long-term	806	830
CECONY [Member] | Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - long-term	280	334
CECONY [Member] | Revenue Taxes [Member]
Regulatory assets
Regulatory assets - long-term	203	196
CECONY [Member] | Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	109	211
CECONY [Member] | Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - long-term	103	74
CECONY [Member] | Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - long-term	73	83
CECONY [Member] | Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - long-term	58	62
CECONY [Member] | Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - long-term	28	28
CECONY [Member] | Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	282	322
CECONY [Member] | Workers' Compensation [Member]
Regulatory assets
Regulatory assets - long-term	12	12
CECONY [Member] | Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - long-term	8	7
CECONY [Member] | Other [Member]
Regulatory assets
Regulatory assets - long-term	175	162
CECONY [Member] | Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - current	14	4
CECONY [Member] | Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	$ 7	$ 22