Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 212
Makeup of net credit exposure independent system operators	92
Makeup of net credit exposure with commodity exchange brokers	108
Makeup of net credit exposure with investment-grade counterparties	8
Makeup of net credit exposure non-investment grade/non-rated counterparties	4
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 35