Fair Value Measurements - Schedule of Commodity Derivatives (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013
Electricity [Member]
Financial Instruments [Line Items]
Valuation Techniques	Discounted Cash Flow
Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Valuation Techniques	Discounted Cash Flow
Transmission Congestion Contracts / Financial Transmission Rights [Member]
Financial Instruments [Line Items]
Valuation Techniques	Discounted Cash Flow
Minimum [Member] | Present Value Factor [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	2.64%
Minimum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	(5.80%)
Minimum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	(102.40%)
Minimum [Member] | Inter-Zonal Forward Price Curves and for Historical Zonal Losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	1.56%
Unobservable Inputs Range	$ 1.56
Minimum [Member] | Forward energy prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	24
Minimum [Member] | Forward capacity prices [Member] | Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	119
Maximum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	42.40%
Maximum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	49.20%
Maximum [Member] | Inter-Zonal Forward Price Curves and for Historical Zonal Losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	2.16%
Unobservable Inputs Range	2.16
Maximum [Member] | Forward energy prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	99
Maximum [Member] | Forward capacity prices [Member] | Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	248
Level 3 [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(14,000,000)
Level 3 [Member] | Electricity [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(6,000,000)
Level 3 [Member] | Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(17,000,000)
Level 3 [Member] | Transmission Congestion Contracts / Financial Transmission Rights [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	9,000,000
CECONY [Member] | Transmission Congestion Contracts [Member]
Financial Instruments [Line Items]
Valuation Techniques	Discounted Cash Flow
CECONY [Member] | Minimum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	(5.80%)
CECONY [Member] | Minimum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	(102.40%)
CECONY [Member] | Maximum [Member] | Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	42.40%
CECONY [Member] | Maximum [Member] | Discount to Adjust Auction Prices for Historical Monthly Realized Settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in percentage	49.20%
CECONY [Member] | Level 3 [Member] | Transmission Congestion Contracts [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	$ 8,000,000