Regulatory Matters - Additional Information (Detail) Customer in Millions	1 Months Ended		9 Months Ended
	Oct. 31, 2012 Customer	Feb. 28, 2009 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Public Utilities, General Disclosures [Line Items]
Annual electric revenue subject to potential refund		$ 249,000,000
Annual gas revenue subject to potential refund		32,000,000
Annual steam revenue subject to potential refund		$ 6,000,000
Potential estimated refund to customers			$ 1,889,000,000
Construction expenditures overcharge consultant estimate			208,000,000
Expected credit to customers			116,000,000
Return on capital expenditures, not recoverable (up to)			55,000,000
Regulatory liabilities			1,940,000,000	$ 1,993,000,000
Prudence proceeding
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities			100,000,000	105,000,000
CECONY
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities			1,755,000,000	1,837,000,000
Response and restoration costs			509,000,000
Capital expenditures			148,000,000
CECONY | Prudence proceeding
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities			100,000,000	$ 105,000,000
O&R
Public Utilities, General Disclosures [Line Items]
Response and restoration costs			91,000,000
Capital expenditures			$ 15,000,000
Storm Damage
Public Utilities, General Disclosures [Line Items]
Number of customers interrupted electric distribution service | Customer	1.4