Regulatory Matters - Summary of Rate Plan (CECONY-Electric) (Detail) (CECONY [Member], Electric [Member], Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Amortizations to income of net regulatory (assets) liabilities	$ 123
Retention of annual transmission congestion revenues	90
Average rate base	18,282
Weighted average cost of capital (after-tax)	6.91%
Authorized return on common equity	9.00%
Earnings sharing percentage	9.60%
Cost of long-term debt	5.09%
Common equity ratio	48.00%
Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties	400
T&D [Member]
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations	17,929
Storm Hardening [Member]
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations	268
Other [Member]
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations	$ 2,069