Regulatory Matters - Other Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			6 Months Ended
	Jun. 30, 2013	May 31, 2013	Jan. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Regulatory Assets And Liabilities [Line Items]
Revenue from electric				$ 249
Revenue from gas				32
Revenue from steam service				6
Potential refund to customers				1,246
Overcharges of construction expenditures				208
Refund amount	16			16
Regulatory asset	9,360			9,360	9,779
Minimum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Refund amount	16			16
Maximum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Overcharges of construction expenditures				208
Storm Damage [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Number of customers interrupted electric distribution service				1,400,000
Rockland Electric Company [Member] | Storm costs [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory asset	29			29
Capital expenditures				6
CECONY [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Electric rate increase	425		375
Gas rate increase	26		25
Steam rate decrease		28	5
Return on common equity	10.10%	8.70%	10.35%
Common equity ratio	50.00%	48.00%	50.00%
Electric rate decreases		187
Gas rate decreases		122
Steam rate increase	11
Response and restoration costs				457
Capital expenditure				141
Regulatory asset	8,646			8,646	9,032
O&R [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Response and restoration costs				93
Capital expenditure				$ 15