Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 146	$ 146
Makeup of net credit exposure, with investment-grade counterparties	38	38
Makeup of net credit exposure with commodity exchange brokers	37	37
Makeup of net credit exposure independent system operators		69
Makeup of net credit exposure non-investment grade/non-rated counterparties		2
Number of Capacity Contracts		1,188
Interest Rate Swap [Member]
Investment Holdings [Line Items]
Derivative, fixed interest rate	6.09%	6.09%
Unrealized gain (loss) on derivatives		(4)
Increase in the fair value of derivative	1	2
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 15	$ 15
Number of Capacity Contracts		620