Regulatory Matters - Other Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 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	1,174
Overcharges of construction expenditures	208
Regulatory liability	15
Regulatory asset	9,516	9,779
Storm Damage [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Number of customers Interrupted electric distribution service	1,400,000
CECONY [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Response and restoration costs	431
Capital expenditure	113
Regulatory asset	8,802	9,032
O&R [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Response and restoration costs	90
Capital expenditure	15
Rockland Electric Company [Member] | Storm costs [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory asset	27
Capital expenditures	$ 6