Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 132	$ 53
Gross Amounts Offset in the Statement of Financial Position	(27)	(3)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	105	50
Net Amount	105	50
Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(110)	(113)
Gross Amounts Offset in the Statement of Financial Position	100	98
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(10)	(15)
Net Amount	(10)	(15)
Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	242	166
Gross Amounts Offset in the Statement of Financial Position	(127)	(101)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	115	65
Net Amount	115	65
Financial Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
Financial Instruments [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
Financial Instruments [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
Cash Collateral Received [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
Cash Collateral Received [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
Cash Collateral Received [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	46	(10)
Gross Amounts Offset in the Statement of Financial Position	(2)	5
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	44	(5)
Net Amount	44	(5)
CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(49)	(51)
Gross Amounts Offset in the Statement of Financial Position	43	37
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(6)	(14)
Net Amount	(6)	(14)
CECONY [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	95	41
Gross Amounts Offset in the Statement of Financial Position	(45)	(32)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	50	9
Net Amount	50	9
CECONY [Member] | Financial Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member] | Financial Instruments [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member] | Financial Instruments [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member] | Cash Collateral Received [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member] | Cash Collateral Received [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	0	0
CECONY [Member] | Cash Collateral Received [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position	$ 0	$ 0