Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accrued Liabilities:
Accrued Liabilities	$ 758	$ 764
Regulatory assets	8,972	9,304
Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	678	684
Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	80	80
Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	758	764
Regulatory assets	911	925
CECONY [Member]
Accrued Liabilities:
Accrued Liabilities	662	666
Regulatory assets	8,302	8,613
CECONY [Member] | Manufactured Gas Plant Sites [Member]
Accrued Liabilities:
Accrued Liabilities	582	587
CECONY [Member] | Other Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	80	79
CECONY [Member] | Superfund Sites [Member]
Accrued Liabilities:
Accrued Liabilities	662	666
Regulatory assets	$ 808	$ 820