Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accrued Liabilities:
Regulatory assets	$ 9,779	$ 9,595
Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	462	422
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	83	67
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	545	489
Regulatory assets	730	681
CECONY [Member]
Accrued Liabilities:
Regulatory assets	9,032	8,890
CECONY [Member] | Manufactured gas plant sites [Member]
Accrued Liabilities:
Accrued Liabilities	351	307
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	82	66
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	433	373
Regulatory assets	$ 615	$ 564