Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Site Contingency [Line Items]
Regulatory assets	$ 9,304	$ 7,230
Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	8	8
Regulatory assets	8	8
Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	83	87
Regulatory assets	8	12
CECONY [Member]
Site Contingency [Line Items]
Regulatory assets	8,613	6,665
CECONY [Member] | Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	7	7
Regulatory assets	7	7
CECONY [Member] | Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	78	82
Regulatory assets	$ 8	$ 12