Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 137	$ 53
Gross Amounts Offset in the Statement of Financial Position	(5)	(3)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	132	50
Net Amount	132	50
Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(95)	(113)
Gross Amounts Offset in the Statement of Financial Position	89	98
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(6)	(15)
Net Amount	(6)	(15)
Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	232	166
Gross Amounts Offset in the Statement of Financial Position	(94)	(101)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	138	65
Net Amount	138	65
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	47	(10)
Gross Amounts Offset in the Statement of Financial Position	(1)	5
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	46	(5)
Net Amount	46	(5)
CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(36)	(51)
Gross Amounts Offset in the Statement of Financial Position	33	37
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(3)	(14)
Net Amount	(3)	(14)
CECONY [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	83	41
Gross Amounts Offset in the Statement of Financial Position	(34)	(32)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	49	9
Net Amount	49	9
Financial Instruments [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
Financial Instruments [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments [Member] | CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments [Member] | CECONY [Member] | Derivative Assets [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 Liabilities [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] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash Collateral Received [Member] | CECONY [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash Collateral Received [Member] | CECONY [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position