Fair Value Measurements - Schedule of Commodity Derivatives (Details) $ in Millions	Sep. 30, 2020 USD ($) $ / MWh $ / kW-month	Dec. 31, 2019 USD ($)
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 670	$ 550
CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	552	498
Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	4	2
Level 3 | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	1	$ 1
Level 3 | Electricity | Forward energy prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (2)
Level 3 | Electricity | Forward energy prices | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	12.75
Level 3 | Electricity | Forward energy prices | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	65.77
Level 3 | Electricity | Forward energy prices | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (1)
Level 3 | Electricity | Forward energy prices | CECONY | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	24.25
Level 3 | Electricity | Forward energy prices | CECONY | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / MWh	65.77
Level 3 | Electricity | Forward capacity prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (16)
Level 3 | Electricity | Forward capacity prices | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	0.13
Level 3 | Electricity | Forward capacity prices | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	7.45
Level 3 | Electricity | Forward capacity prices | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (9)
Level 3 | Electricity | Forward capacity prices | CECONY | Minimum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	0.19
Level 3 | Electricity | Forward capacity prices | CECONY | Maximum
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per unit) | $ / kW-month	7.45
Level 3 | 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
Level 3 | 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	(5.73)
Level 3 | 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	9.64
Level 3 | 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
Level 3 | 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	0.15
Level 3 | 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	3.13
Level 3 | Commodity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (17)
Level 3 | Commodity | CECONY
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ (9)