Derivative Instruments and Hedging Activities - Hedged Volume of Derivative Transactions (Details) gal in Thousands	Dec. 31, 2017 MW MMBTU gal MWh
Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	31,741,652
Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	10,275
Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	177,433,144
Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	3,780
CECONY | Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	29,696,600
CECONY | Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	5,100
CECONY | Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	169,790,000
CECONY | Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	3,780