Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Site Contingency [Line Items]
Regulatory assets	$ 6,566.0	$ 7,230.0
Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	8	8
Regulatory assets	8.0	8.0
Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	86	87
Regulatory assets	10.0	12.0
CECONY [Member]
Site Contingency [Line Items]
Regulatory assets	6,023.0	6,665.0
CECONY [Member] | Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	7	7
Regulatory assets	7.0	7.0
CECONY [Member] | Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	81	82
Regulatory assets	$ 10.0	$ 12.0