Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012 CECONY [Member]	Dec. 31, 2011 CECONY [Member]
Accumulated OCI at December 31, 2012	$ (53)	$ (9)	$ (9)
OCI before reclassifications	1
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax of $1 and $- for Con Edison and CECONY, respectively (a) (b)	2
Net OCI at March 31, 2013	3
Accumulated OCI, net of taxes, at March 31, 2013 (b)	$ (50)	$ (9)	$ (9)