Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets - long-term	$ 9,484	$ 9,705
Regulatory assets - current	32	74
Regulatory liabilities - long-term	1,234	1,202
Total Regulatory Assets	9,516	9,779
Regulatory liabilities - current	132	183
Total Regulatory Liabilities	1,366	1,385
Unrecognized pension and other postretirement costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	5,443	5,677
Future income tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1,952	1,922
Environmental remediation costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	721	730
Deferred storm costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	443	432
Pension and other postretirement benefits deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	161	183
Revenue taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	179	176
Net electric deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	97	102
Unamortized loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	71	74
Surcharge for New York State assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	117	73
Deferred derivative losses - long-term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	32	40
O&R transition bond charges [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	37	39
Preferred stock redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	29	29
Recoverable energy costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term		23
Regulatory assets - current	1	5
Workers' compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	20	19
Property tax reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	16	16
Regulatory liabilities - long-term	237	187
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	166	170
Deferred derivative losses - current [Member]
Regulatory Assets [Line Items]
Regulatory assets - current	31	69
Allowance for cost removal less salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	508	503
Net unbilled revenue deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	64	136
World Trade Center settlement proceeds [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	62	62
Long-term interest rate reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	77	62
Carrying charges on transmission and distribution net plant [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	25	31
Expenditure prudence proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	15	14
Gas line losses [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	14	14
Energy efficiency programs [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	6	5
Other [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	226	188
Refundable energy costs - current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	100	82
Revenue decoupling mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	23	72
Electric Surcharge offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current		29
Deferred derivative gains - current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	9
CECONY [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	8,774	8,972
Regulatory assets - current	28	60
Regulatory liabilities - long-term	1,110	1,077
Total Regulatory Assets	8,802	9,032
Regulatory liabilities - current	98	145
Total Regulatory Liabilities	1,208	1,222
CECONY [Member] | Unrecognized pension and other postretirement costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	5,191	5,407
CECONY [Member] | Future income tax [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	1,851	1,831
CECONY [Member] | Environmental remediation costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	607	615
CECONY [Member] | Deferred storm costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	330	309
CECONY [Member] | Pension and other postretirement benefits deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	131	154
CECONY [Member] | Revenue taxes [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	172	170
CECONY [Member] | Net electric deferrals [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	97	102
CECONY [Member] | Unamortized loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	68	70
CECONY [Member] | Surcharge for New York State assessment [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	111	68
CECONY [Member] | Deferred derivative losses - long-term [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	14	20
CECONY [Member] | Preferred stock redemption [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	29	29
CECONY [Member] | Recoverable energy costs [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term		23
CECONY [Member] | Workers' compensation [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	20	19
CECONY [Member] | Property tax reconciliation [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	237	187
CECONY [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets - long-term	153	155
CECONY [Member] | Deferred derivative losses - current [Member]
Regulatory Assets [Line Items]
Regulatory assets - current	28	60
CECONY [Member] | Allowance for cost removal less salvage [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	424	420
CECONY [Member] | Net unbilled revenue deferrals [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	64	136
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	77	62
CECONY [Member] | Carrying charges on transmission and distribution net plant [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	10	13
CECONY [Member] | Expenditure prudence proceeding [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	15	14
CECONY [Member] | Gas line losses [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	14	14
CECONY [Member] | Energy efficiency programs [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	6	6
CECONY [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - long-term	201	163
CECONY [Member] | Refundable energy costs - current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	70	48
CECONY [Member] | Revenue decoupling mechanism [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	20	68
CECONY [Member] | Electric Surcharge offset [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current		29
CECONY [Member] | Deferred derivative gains - current [Member]
Regulatory Assets [Line Items]
Regulatory liabilities - current	$ 8