Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 116	$ 53
Gross Amounts Offset in the Statement of Financial Position	(24)	(3)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	92	50
Net Amount	92	50
Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(124)	(113)
Gross Amounts Offset in the Statement of Financial Position	114	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)	240	166
Gross Amounts Offset in the Statement of Financial Position	(138)	(101)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	102	65
Net Amount	102	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)	34	(10)
Gross Amounts Offset in the Statement of Financial Position	2	5
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	36	(5)
Net Amount	36	(5)
CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(69)	(51)
Gross Amounts Offset in the Statement of Financial Position	65	37
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(4)	(14)
Net Amount	(4)	(14)
CECONY [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	103	41
Gross Amounts Offset in the Statement of Financial Position	(63)	(32)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	40	9
Net Amount	40	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