Short-Term Borrowing - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Commercial paper, outstanding	$ 1,160,000,000	$ 800,000,000
Weighted average interest rate (percent)	0.30%	0.40%
Loans outstanding under credit agreement	$ 0	$ 0
Letters of credit outstanding under the Credit Agreement	56,000,000	11,000,000
CECONY
Short-term Debt [Line Items]
Commercial paper, outstanding	$ 649,000,000	$ 450,000,000
Weighted average interest rate (percent)	0.30%	0.40%
Letters of credit outstanding under the Credit Agreement	$ 11,000,000	$ 11,000,000