Fair Value Measurements - Schedule of Commodity Derivatives (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) $ / kW-month $ / MWh
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
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	1.60%
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	(5.10)
Minimum | Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / MWh	15.32
Minimum | Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / kW-month	0.58
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	62.10%
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	82.20%
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	2.25
Maximum | Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / MWh	74.99
Maximum | Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) | $ / kW-month	8.10
Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives | $	$ (4)
Level 3 | Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives | $	(7)
Level 3 | Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives | $	$ 3
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	1.60%
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	62.10%
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	82.20%
CECONY | Level 3 | Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives | $	$ 2