Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 154
Makeup of net credit exposure with commodity exchange brokers	71
Makeup of net credit exposure independent system operators	69
Makeup of net credit exposure with investment-grade counterparties	7
Makeup of net credit exposure non-investment grade/non-rated counterparties	7
CECONY
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 17