Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Con Edison [Member]
Accrued Liabilities:
Regulatory assets	$ 8,974	$ 9,501
Con Edison [Member] | Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued liabilities	466	422
Con Edison [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued liabilities	71	67
Con Edison [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued liabilities	537	489
Regulatory assets	722	681
CECONY [Member]
Accrued Liabilities:
Regulatory assets	8,347	8,801
CECONY [Member] | Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued liabilities	352	307
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued liabilities	71	66
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued liabilities	423	373
Regulatory assets	$ 607	$ 564