Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accrued Liabilities:
Regulatory assets	$ 9,236	$ 9,779
Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	439	462
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	73	83
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	512	545
Regulatory assets	704	730
CECONY [Member]
Accrued Liabilities:
Regulatory assets	8,539	9,032
CECONY [Member] | Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	332	351
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	71	82
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	403	433
Regulatory assets	$ 591	$ 615