Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Accrued Liabilities:
Accrued Liabilities	$ 746	$ 764
Regulatory assets	8,512	9,304
Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	666	684
Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	80	80
Superfund Sites
Accrued Liabilities:
Accrued Liabilities	746	764
Regulatory assets	884	925
CECONY
Accrued Liabilities:
Accrued Liabilities	651	666
Regulatory assets	7,873	8,613
CECONY | Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	572	587
CECONY | Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	79	79
CECONY | Superfund Sites
Accrued Liabilities:
Accrued Liabilities	651	666
Regulatory assets	$ 784	$ 820