Related Party Transactions - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CECONY [Member]
Related Party Transaction [Line Items]
Sale of natural gas	$ 80,000,000	$ 72,000,000	$ 54,000,000
Funding limit of CECONY to O&R	250,000,000
O&R [Member]
Related Party Transaction [Line Items]
Outstanding loans to O&R	$ 0	$ 0