Fair Value Measurements - Schedule of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 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
Natural Gas [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	9.60%
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	32.30%
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	(2.66)
Minimum [Member] | Forward Energy Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	22.59
Minimum [Member] | Forward Capacity Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	1.00
Minimum [Member] | Forward Gas Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	(1.64)
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	56.10%
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	16.49
Maximum [Member] | Forward Energy Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	120
Maximum [Member] | Forward Capacity Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	8.80
Maximum [Member] | Forward Gas Prices [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range	5.00
Level 3 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	20
Level 3 [Member] | Electricity [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	1
Level 3 [Member] | Natural Gas [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	2
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	17
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	9.60%
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	32.30%
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	56.10%
CECONY [Member] | Level 3 [Member] | Transmission Congestion Contracts [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives	13