Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 53	$ (90)
Gross Amounts Offset in the Statement of Financial Position	(3)	47
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	50	(43)
Net Amount	50	(43)
Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	166	86
Gross Amounts Offset in the Statement of Financial Position	(101)	(57)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	65	29
Net Amount	65	29
Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(113)	(176)
Gross Amounts Offset in the Statement of Financial Position	98	104
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(15)	(72)
Net Amount	(15)	(72)
Financial Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash Collateral Received [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash Collateral Received [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash Collateral Received [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(10)	(56)
Gross Amounts Offset in the Statement of Financial Position	5	29
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(5)	(27)
Net Amount	(5)	(27)
CECONY [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	41	27
Gross Amounts Offset in the Statement of Financial Position	(32)	(15)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	9	12
Net Amount	9	12
CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(51)	(83)
Gross Amounts Offset in the Statement of Financial Position	37	44
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(14)	(39)
Net Amount	(14)	(39)
CECONY [Member] | Financial Instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member] | Financial Instruments [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member] | Financial Instruments [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member] | Cash Collateral Received [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member] | Cash Collateral Received [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
CECONY [Member] | Cash Collateral Received [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position