Environmental Matters - Environmental Remediation Costs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accrued Environmental Remediation Costs [Line Items]
Remediation costs incurred	$ 10	$ 3	$ 35	$ 18
Insurance recoveries received
CECONY [Member]
Accrued Environmental Remediation Costs [Line Items]
Remediation costs incurred	10	1	30	15
Insurance recoveries received