Fair Value Measurements - Schedule of Commodity Derivatives (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Electricity Swaps [Member]
Financial Instruments [Line Items]
Valuation Techniques	Discounted Cash Flow
Electricity Wholesale Contract [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] | Forward price escalator [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	0.00%
Minimum [Member] | Present value factor [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	1.66%
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] | Forward Energy Prices And Zonal Losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	0.01
Minimum [Member] | Forward prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	28
Minimum [Member] | Forward Prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	29
Minimum [Member] | New Jersey solar renewable energy credit [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	90
Minimum [Member] | Forward capacity prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	166
Maximum [Member] | Forward price escalator [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	3.00%
Maximum [Member] | Discount to adjust auction prices for inter-zonal forward price curves [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	38.00%
Maximum [Member] | Discount to adjust auction prices for historical monthly realized settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	31.00%
Maximum [Member] | Forward Energy Prices And Zonal Losses [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	12.00
Maximum [Member] | Forward prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	82
Maximum [Member] | Forward Prices [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range	48
Level 3 [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(5)
Level 3 [Member] | Electricity Swaps [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(5)
Level 3 [Member] | Electricity Wholesale Contract [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(1)
Level 3 [Member] | Standard Offer Capacity Agreements [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	(12)
Level 3 [Member] | Transmission Congestion Contracts / Financial Transmission Rights [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	13
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	38.00%
CECONY [Member] | Maximum [Member] | Discount to adjust auction prices for historical monthly realized settlements [Member]
Financial Instruments [Line Items]
Unobservable Inputs Range in Percentage	31.00%
CECONY [Member] | Level 3 [Member] | Transmission Congestion Contracts [Member]
Financial Instruments [Line Items]
Fair Value of commodity derivatives	10