Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax	$ 18	$ (15)
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax	(4)	(5)
CECONY [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI before reclassifications, Tax	4	(1)
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax	$ (1)	$ (1)