Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Accrued Liabilities:
Accrued Liabilities	$ 734	$ 749
Regulatory assets	6,566.0	7,230.0
Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	655	665
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	79	84
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	734	749
Regulatory assets	899.0	938.0
CECONY [Member]
Accrued Liabilities:
Accrued Liabilities	636	644
Regulatory assets	6,023.0	6,665.0
CECONY [Member] | Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	558	562
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	78	82
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	636	644
Regulatory assets	$ 795.0	$ 830.0