Fair Value Measurements - Schedule of Commodity Derivatives (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) $ / MWh $ / kW-month
Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Forward Capacity Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Minimum | Discount to adjust auction prices for inter-zonal forward price curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	52.80%
Minimum | Discount/premium to adjust auction prices for historical monthly realized settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	(44.90%)
Minimum | Inter-zonal forward price curves adjusted for historical zonal losses
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per kW-month) | $ / MWh	(0.14)
Minimum | Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / MWh	19.75
Minimum | Forward Capacity Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / kW-month	2.68
Maximum | Discount to adjust auction prices for inter-zonal forward price curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	59.40%
Maximum | Discount/premium to adjust auction prices for historical monthly realized settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	58.90%
Maximum | Inter-zonal forward price curves adjusted for historical zonal losses
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per kW-month) | $ / MWh	2.82
Maximum | Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / MWh	80
Maximum | Forward Capacity Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / kW-month	9.45
Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1
Level 3 | Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	1
Level 3 | Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	(1)
Level 3 | Forward Capacity Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1
CECONY | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
CECONY | Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
CECONY | Minimum | Discount to adjust auction prices for inter-zonal forward price curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	52.80%
CECONY | Minimum | Discount/premium to adjust auction prices for historical monthly realized settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	(44.90%)
CECONY | Minimum | Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / MWh	21.10
CECONY | Maximum | Discount to adjust auction prices for inter-zonal forward price curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	59.40%
CECONY | Maximum | Discount/premium to adjust auction prices for historical monthly realized settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	58.90%
CECONY | Maximum | Forward Energy Prices | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH) | $ / MWh	80
CECONY | Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 0
CECONY | Level 3 | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	(1)
CECONY | Level 3 | Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1