Environmental Matters - Accrued Liabilities and Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Accrued Liabilities	$ 737	$ 753
Regulatory assets	4,333	7,124
Manufactured gas plant sites
Site Contingency [Line Items]
Accrued Liabilities	651	664
Other Superfund Sites
Site Contingency [Line Items]
Accrued Liabilities	86	89
Superfund Sites
Site Contingency [Line Items]
Accrued Liabilities	737	753
Regulatory assets	793	823
CECONY
Site Contingency [Line Items]
Accrued Liabilities	637	655
Regulatory assets	3,925	6,563
CECONY | Manufactured gas plant sites
Site Contingency [Line Items]
Accrued Liabilities	551	567
CECONY | Other Superfund Sites
Site Contingency [Line Items]
Accrued Liabilities	86	88
CECONY | Superfund Sites
Site Contingency [Line Items]
Accrued Liabilities	637	655
Regulatory assets	$ 677	$ 711