Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Electric) (Parenthetical) (Detail) (Electric Transmission [Member], CECONY [Member], USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Mar. 31, 2013	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Potential refund from customers		$ 249
Elimination of temporary base rate increase through credits			134
Deferrals for property taxes limitation from rates	80.00%	90.00%
Annual maximum property tax adjustment allowed on required rate of return		Not more than a 10 basis point
PJM Interconnection L.L.C. [Member]
Public Utilities, General Disclosures [Line Items]
Recovery of charges pursuant to tariff of PJM		20
Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Base rate change deferral regulatory liability impact				30
Storm Reserve [Member]
Public Utilities, General Disclosures [Line Items]
Revenue requirement		21
Storm Reserve [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Revenue requirement				$ 21