Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Accrued Liabilities:
Accrued Liabilities	$ 738	$ 749
Regulatory assets	6,806.0	7,230.0
Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	658	665
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	80	84
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	738	749
Regulatory assets	911.0	938.0
CECONY [Member]
Accrued Liabilities:
Accrued Liabilities	637	644
Regulatory assets	6,256.0	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	79	82
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	637	644
Regulatory assets	$ 806.0	$ 830.0