Fair Value Measurements - Schedule of Commodity Derivatives (Detail) (USD $)	6 Months Ended
	Jun. 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	2.49%
Minimum [Member] | Discount to adjust auction prices for inter-zonal forward price curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	17.50%
Minimum [Member] | Discount to adjust auction prices for historical monthly realized settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	8.50%
Minimum [Member] | Inter-zonal forward price curves and for historical zonal losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	$ 0.54
Minimum [Member] | Forward energy prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	20
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	49.00%
Maximum [Member] | Inter-zonal forward price curves and for historical zonal losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	12.68
Maximum [Member] | Forward energy prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	129
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	(6,000,000)
Level 3 [Member] | Electricity [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(2,000,000)
Level 3 [Member] | Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(15,000,000)
Level 3 [Member] | Transmission Congestion Contracts / Financial Transmission Rights [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	11,000,000
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	17.50%
CECONY [Member] | Minimum [Member] | Discount to adjust auction prices for historical monthly realized settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	8.50%
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	49.00%
CECONY [Member] | Level 3 [Member] | Transmission Congestion Contracts [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	$ 8,000,000