Derivative Instruments and Hedging Activities - Hedged Volume of Derivative Transactions (Details) gal in Thousands	Sep. 30, 2018 gal MMBTU MW MWh
Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	30,590,984
Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	8,134
Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	195,821,882
Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	2,688
CECONY | Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	27,983,800
CECONY | Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	3,300
CECONY | Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	182,820,000
CECONY | Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	2,688