Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax	$ 1	$ 1
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax	1	1
CECONY [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax