Environmental Matters - Accrued Liabilities and Regulatory Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Site Contingency [Line Items]
Accrued liabilities	$ 720	$ 737
Regulatory assets	3,975	4,333
Manufactured gas plant sites
Site Contingency [Line Items]
Accrued liabilities	639	651
Other Superfund Sites
Site Contingency [Line Items]
Accrued liabilities	81	86
Superfund Sites
Site Contingency [Line Items]
Accrued liabilities	720	737
Regulatory assets	754	793
CECONY
Site Contingency [Line Items]
Accrued liabilities	626	637
Regulatory assets	3,595	3,925
CECONY | Manufactured gas plant sites
Site Contingency [Line Items]
Accrued liabilities	546	551
CECONY | Other Superfund Sites
Site Contingency [Line Items]
Accrued liabilities	80	86
CECONY | Superfund Sites
Site Contingency [Line Items]
Accrued liabilities	626	637
Regulatory assets	$ 653	$ 677