Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 110	$ 110
Makeup of net credit exposure, with investment-grade counterparties	30	30
Makeup of net credit exposure with commodity exchange brokers	37	37
Makeup of net credit exposure independent system operators		42
Makeup of net credit exposure non-investment grade or non-rated counterparties		1
Number of Capacity Contracts		1,187
Interest Rate Swap [Member]
Investment Holdings [Line Items]
Derivative, fixed interest rate	6.09%	6.09%
Unrealized gain (loss) on derivatives		(3)
Increase in the fair value of derivative	1	3
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 14	$ 14
Number of Capacity Contracts		628