Fair Value Measurements - Schedule of Commodity Derivatives (Details) - Level 3 $ in Millions	Sep. 30, 2018 USD ($) $ / kW-month $ / MWh $ / Dt
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (10)
CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	0
Electricity | Forward energy prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (7)
Electricity | Forward energy prices | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	18.33
Electricity | Forward energy prices | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	79.20
Electricity | Forward capacity prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives
Electricity | Forward capacity prices | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	1.70
Electricity | Forward capacity prices | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	8.84
Electricity | Forward capacity prices | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (1)
Electricity | Forward capacity prices | CECONY | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	1.70
Electricity | Forward capacity prices | CECONY | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	7.20
Natural Gas | Forward natural gas prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (4)
Natural Gas | Forward natural gas prices | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / Dt	0.96
Natural Gas | Forward natural gas prices | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / Dt	10.51
Transmission Congestion Contracts/Financial Transmission Rights | Inter-zonal forward price curves adjusted for historical zonal losses
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1
Transmission Congestion Contracts/Financial Transmission Rights | Inter-zonal forward price curves adjusted for historical zonal losses | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	0.37
Transmission Congestion Contracts/Financial Transmission Rights | Inter-zonal forward price curves adjusted for historical zonal losses | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	5.45
Transmission Congestion Contracts | Inter-zonal forward price curves adjusted for historical zonal losses | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1
Transmission Congestion Contracts | Inter-zonal forward price curves adjusted for historical zonal losses | CECONY | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	0.45
Transmission Congestion Contracts | Inter-zonal forward price curves adjusted for historical zonal losses | CECONY | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	4.18