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, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ (44)	$ (90)
Gross Amounts Offset in the Statement of Financial Position	22	47
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(22)	(43)
Net Amount	(22)	(43)
Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	80	86
Gross Amounts Offset in the Statement of Financial Position	(52)	(57)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	28	29
Net Amount	28	29
Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(124)	(176)
Gross Amounts Offset in the Statement of Financial Position	74	104
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(50)	(72)
Net Amount	(50)	(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)	(31)	(56)
Gross Amounts Offset in the Statement of Financial Position	15	29
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(16)	(27)
Net Amount	(16)	(27)
CECONY [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	27	27
Gross Amounts Offset in the Statement of Financial Position	(18)	(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)	(58)	(83)
Gross Amounts Offset in the Statement of Financial Position	33	44
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(25)	(39)
Net Amount	(25)	(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