Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax	$ 0	$ 0	$ 1	$ 1
Amounts reclassified from accumulated OCI related to pension and other postretirement benefit plan liabilities, Tax	1	1	3	3
CECONY [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax	0	0	0	0
Amounts reclassified from accumulated OCI related to pension and other postretirement benefit plan liabilities, Tax	$ 0	$ 0	$ 0	$ 0