Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accrued Liabilities:
Accrued Liabilities	$ 764	$ 749
Regulatory assets	9,304	7,230
Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	684	665
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	80	84
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	764	749
Regulatory assets	925	938
CECONY [Member]
Accrued Liabilities:
Accrued Liabilities	666	644
Regulatory assets	8,613	6,665
CECONY [Member] | Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	587	562
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	79	82
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	666	644
Regulatory assets	$ 820	$ 830