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, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 7	$ (90)
Gross Amounts Offset in the Statement of Financial Position	14	47
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	21	(43)
Net Amount	21	(43)
Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	121	86
Gross Amounts Offset in the Statement of Financial Position	(64)	(57)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	57	29
Net Amount	57	29
Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(114)	(176)
Gross Amounts Offset in the Statement of Financial Position	78	104
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(36)	(72)
Net Amount	(36)	(72)
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(11)	(56)
Gross Amounts Offset in the Statement of Financial Position	15	29
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	4	(27)
Net Amount	4	(27)
CECONY [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	40	27
Gross Amounts Offset in the Statement of Financial Position	(19)	(15)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	21	12
Net Amount	21	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	34	44
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(17)	(39)
Net Amount	(17)	(39)
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
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 assets [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
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
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 assets [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