Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014 Contract
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 141	$ 141
Makeup of net credit exposure independent system operators		47
Makeup of net credit exposure with commodity exchange brokers	49	49
Makeup of net credit exposure with investment-grade counterparties	43	43
Makeup of net credit exposure non-investment grade/non-rated counterparties		2
Number of contracts		1,285
Terminate date of interest rate swap		October 2014
Increase in the fair value of interest rate swap	1	1
Interest Rate Swap [Member]
Investment Holdings [Line Items]
Derivative, fixed interest rate	6.09%	6.09%
Unrealized gain (loss) on derivatives		(1)
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	26	26
Makeup of net credit exposure with commodity exchange brokers	15	15
Makeup of net credit exposure with investment-grade counterparties	$ 11	$ 11
Number of contracts		576