Fair Value Measurements - Schedule of Commodity Derivatives (Detail) - Jun. 30, 2015 $ in Millions	USD ($) $ / kW-month $ / MWh $ / Dt
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Natural Gas
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	40.80%
Minimum | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	37.50%
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	(2.57)
Minimum | Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / MWh	18.25
Minimum | Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / kW-month	3.70
Minimum | Forward Gas Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / Dt	(1.56)
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	57.90%
Maximum | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	60.80%
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	6.62
Maximum | Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / MWh	118.25
Maximum | Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / kW-month	15.26
Maximum | Forward Gas Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / Dt	10.00
Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 13
Level 3 | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	(2)
Level 3 | Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	14
Level 3 | Natural Gas
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 1
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	40.80%
CECONY | Minimum | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	37.50%
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	57.90%
CECONY | Maximum | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	60.80%
CECONY | Level 3 | Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 11