Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Accrued Liabilities:
Accrued Liabilities	$ 752	$ 765
Regulatory assets	7,663	8,228
Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	664	679
Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	88	86
Superfund Sites
Accrued Liabilities:
Accrued Liabilities	752	765
Regulatory assets	823	904
CECONY
Accrued Liabilities:
Accrued Liabilities	661	665
Regulatory assets	7,097	7,603
CECONY | Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	574	579
CECONY | Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	87	86
CECONY | Superfund Sites
Accrued Liabilities:
Accrued Liabilities	661	665
Regulatory assets	$ 720	$ 800