Derivative Instruments and Hedging Activities - Hedged Volume of Derivative Transactions (Details) gal in Thousands	Sep. 30, 2017 gal MMBTU MW MWh
Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	32,596,372
Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	6,790
Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	166,913,644
Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	672
CECONY | Electric Energy
Derivatives, Fair Value [Line Items]
Notional amount | MWh	30,492,575
CECONY | Capacity
Derivatives, Fair Value [Line Items]
Notional amount | MW	3,000
CECONY | Natural Gas
Derivatives, Fair Value [Line Items]
Notional amount | MMBTU	158,500,000
CECONY | Refined Fuels
Derivatives, Fair Value [Line Items]
Notional amount | gal	672