Short-Term Borrowing - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Commercial paper, outstanding	$ 1,220,000,000	$ 539,000,000
Weighted average interest rate	0.30%	0.30%
Loans outstanding under the credit agreement
Letters of credit outstanding under the credit agreement	29,000,000	131,000,000
Aggregate commitments under the credit agreement	2,100,000,000
Previously Reported [Member]
Short-term Debt [Line Items]
Credit agreement termination date	Oct. 31, 2016
Currently Reported [Member]
Short-term Debt [Line Items]
Credit agreement termination date	Oct. 31, 2017
CECONY [Member]
Short-term Debt [Line Items]
Commercial paper, outstanding	1,042,000,000	421,000,000
Weighted average interest rate	0.30%	0.30%
Letters of credit outstanding under the credit agreement	$ 11,000,000	$ 121,000,000