Derivative Instruments and Hedging Activities - Hedged Volume of Derivative Transactions (Details) gal in Thousands	Mar. 31, 2018 gal MWh MW MMBTU
Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	29,852,714
Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	8,835
Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	172,819,498
Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	2,688
CECONY | Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	27,760,400
CECONY | Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	3,600
CECONY | Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	157,080,000
CECONY | Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	2,688