Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Steam) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Other regulatory liabilities	105
Steam [Member] | CECONY [Member]
Public Utilities, General Disclosures [Line Items]
Potential refund from customers			6
Difference in property taxes	90.00%	80.00%
Maximum impact on return on common equity and deferral of facility relocation expenses	Not more than a 10 basis point
Scenario, Forecast [Member] | Steam [Member] | CECONY [Member]
Public Utilities, General Disclosures [Line Items]
Other regulatory liabilities				$ 8