Fair Value Measurements - Schedule of Commodity Derivatives (Detail) $ in Millions	9 Months Ended
Sep. 30, 2015 USD ($) $ / kW-month $ / MWh
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
Minimum \| Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	40.80%
Minimum \| Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	37.50%
Minimum \| Inter-Zonal Forward Price Curves Adjusted for Historical Zonal Losses
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per kW-month)	(5.30)
Minimum \| Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt)	18.5
Minimum \| Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) \| $ / kW-month	2.96
Maximum \| Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	57.90%
Maximum \| Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	82.20%
Maximum \| Inter-Zonal Forward Price Curves Adjusted for Historical Zonal Losses
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per kW-month)	2.94
Maximum \| Forward Energy Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt)	91.75
Maximum \| Forward Capacity Prices
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range (dollar per MWH/Dt) \| $ / kW-month	15.01
Level 3
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives \| $	$ 4
Level 3 \| Electricity
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives \| $	(5)
Level 3 \| Transmission Congestion Contracts/Financial Transmission Rights
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives \| $	$ 9
CECONY \| Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Valuation Techniques	Discounted Cash Flow
CECONY \| Minimum \| Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	40.80%
CECONY \| Minimum \| Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	37.50%
CECONY \| Maximum \| Discount to Adjust Auction Prices for Inter-Zonal Forward Price Curves
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	57.90%
CECONY \| Maximum \| Discount to Adjust Auction Prices for Historical Monthly Realized Settlements
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Unobservable Inputs Range in percentage	82.20%
CECONY \| Level 3 \| Transmission Congestion Contracts
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Fair Value of commodity derivatives \| $	$ 8

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Description of the inputs and valuation technique(s) used to measure fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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