Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Site Contingency [Line Items]
Regulatory assets	$ 9,236	$ 9,779
Asbestos suits [Member]
Site Contingency [Line Items]
Accrued liability	10	10
Regulatory assets	10	10
Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	91	94
Regulatory assets	16	19
CECONY [Member]
Site Contingency [Line Items]
Regulatory assets	8,539	9,032
CECONY [Member] | Asbestos suits [Member]
Site Contingency [Line Items]
Accrued liability	10	10
Regulatory assets	10	10
CECONY [Member] | Workers Compensation Insurance [Member]
Site Contingency [Line Items]
Accrued liability	86	89
Regulatory assets	$ 16	$ 19