Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets	$ 46	$ 74
Regulatory assets - long-term	9,190	9,705
Regulatory assets - current	46	74
Total Regulatory Assets	9,236	9,779
Regulatory liabilities	117	183
Regulatory liabilities - long-term	1,557	1,202
Regulatory liabilities - current	117	183
Total Regulatory Liabilities	1,674	1,385
Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	5,011	5,677
Future Income Tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	2,035	1,922
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	704	730
Deferred Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	451	432
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	229	183
Revenue Taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	191	176
Surcharge for New York State Assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	119	73
Net Electric Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	88	102
Unamortized Loss on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	67	74
Deferred Derivative Losses - Long-Term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	34	40
O&R Transition Bond Charges [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	34	39
Preferred Stock Redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	28	29
Property Tax Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	20	16
Regulatory liabilities - long-term	290	187
Workers' compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	16	19
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	163	193
Deferred Derivative Losses - Current [Member]
Regulatory Assets [Line Items]
Regulatory assets	45	69
Regulatory assets - current	45	69
Recoverable Energy Costs - Current [Member]
Regulatory Assets [Line Items]
Regulatory assets	1	5
Regulatory assets - current	1	5
Allowance for Cost Removal Less Salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	522	503
Property Tax Refunds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	130	7
Net Unbilled Revenue Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	104	136
Long-Term Interest Rate Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	94	62
World Trade Center Settlement Proceeds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	62	62
Carrying Charges on Transmission and Distribution Net Plant Electric and Steam [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	30	31
Expenditure Prudence Proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	16	14
Other [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	309	200
Refundable Energy Costs - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	64	82
Regulatory liabilities - current	64	82
Revenue Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	51	72
Regulatory liabilities - current	51	72
Deferred Derivative Gains - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	2
Regulatory liabilities - current	2
Electric Surcharge Offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities		29
Regulatory liabilities - current		29
CECONY [Member]
Regulatory Assets [Line Items]
Regulatory assets	42	60
Regulatory assets - long-term	8,497	8,972
Regulatory assets - current	42	60
Total Regulatory Assets	8,539	9,032
Regulatory liabilities	86	145
Regulatory liabilities - long-term	1,431	1,077
Regulatory liabilities - current	86	145
Total Regulatory Liabilities	1,517	1,222
CECONY [Member] | Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	4,779	5,407
CECONY [Member] | Future Income Tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1,929	1,831
CECONY [Member] | Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	591	615
CECONY [Member] | Deferred Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	340	309
CECONY [Member] | Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	201	154
CECONY [Member] | Revenue Taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	182	170
CECONY [Member] | Surcharge for New York State Assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	113	68
CECONY [Member] | Net Electric Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	88	102
CECONY [Member] | Unamortized Loss on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	64	70
CECONY [Member] | Deferred Derivative Losses - Long-Term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	13	20
CECONY [Member] | Preferred Stock Redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	28	29
CECONY [Member] | Property Tax Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	290	187
CECONY [Member] | Workers' compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	16	19
CECONY [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	153	178
CECONY [Member] | Deferred Derivative Losses - Current [Member]
Regulatory Assets [Line Items]
Regulatory assets	42	60
Regulatory assets - current	42	60
CECONY [Member] | Allowance for Cost Removal Less Salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	436	420
CECONY [Member] | Property Tax Refunds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	129	6
CECONY [Member] | Net Unbilled Revenue Deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	104	136
CECONY [Member] | Long-Term Interest Rate Reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	94	62
CECONY [Member] | World Trade Center Settlement Proceeds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	62	62
CECONY [Member] | Carrying Charges on Transmission and Distribution Net Plant Electric and Steam [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	20	13
CECONY [Member] | Expenditure Prudence Proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	16	14
CECONY [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	280	177
CECONY [Member] | Refundable Energy Costs - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	36	48
Regulatory liabilities - current	36	48
CECONY [Member] | Revenue Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	49	68
Regulatory liabilities - current	49	68
CECONY [Member] | Deferred Derivative Gains - Current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities	1
Regulatory liabilities - current	1
CECONY [Member] | Electric Surcharge Offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities		29
Regulatory liabilities - current		$ 29