Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Site Contingency [Line Items]
Regulatory assets	$ 8,972	$ 9,304
Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	8	8
Regulatory assets	8	8
Workers' Compensation [Member]
Site Contingency [Line Items]
Accrued liability	84	83
Regulatory assets	9	8
CECONY [Member]
Site Contingency [Line Items]
Regulatory assets	8,302	8,613
CECONY [Member] | Asbestos Suits [Member]
Site Contingency [Line Items]
Accrued liability	7	7
Regulatory assets	7	7
CECONY [Member] | Workers' Compensation [Member]
Site Contingency [Line Items]
Accrued liability	79	78
Regulatory assets	$ 9	$ 8