Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accrued Liabilities:
Regulatory assets	$ 9,360	$ 9,779
Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	447	462
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	75	83
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	522	545
Regulatory assets	713	730
CECONY [Member]
Accrued Liabilities:
Regulatory assets	8,646	9,032
CECONY [Member] | Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	339	351
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	74	82
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	413	433
Regulatory assets	$ 599	$ 615