Fair Value Measurements - Schedule of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Electricity [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Transmission Congestion Contracts / Financial Transmission Rights [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Minimum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	(5.80%)
Minimum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	6.10%
Minimum [Member] | Inter-Zonal Forward Price Curves Adjusted for Historical Zonal Losses [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	0.08
Minimum [Member] | Forward Energy Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	28.43
Minimum [Member] | Forward Capacity Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	5.39
Maximum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	57.90%
Maximum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	49.10%
Maximum [Member] | Inter-Zonal Forward Price Curves Adjusted for Historical Zonal Losses [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	7.61
Maximum [Member] | Forward Energy Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	170
Maximum [Member] | Forward Capacity Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	17.94
Level 3 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 21
Level 3 [Member] | Electricity [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	9
Level 3 [Member] | Transmission Congestion Contracts / Financial Transmission Rights [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	12
CECONY [Member] | Transmission Congestion Contracts [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
CECONY [Member] | Minimum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	(5.80%)
CECONY [Member] | Minimum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	6.10%
CECONY [Member] | Maximum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	57.90%
CECONY [Member] | Maximum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	49.10%
CECONY [Member] | Level 3 [Member] | Transmission Congestion Contracts [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	$ 11