Derivative Instruments and Hedging Activities - Hedged Volume of Derivative Transactions (Details) gal in Thousands	Jun. 30, 2018 gal MMBTU MW MWh
Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	35,139,532
Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	8,356
Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	184,203,665
Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	1,680
CECONY | Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	32,200,400
CECONY | Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	3,450
CECONY | Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	177,250,000
CECONY | Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	1,680