Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 148
Makeup of net credit exposure independent system operators	45
Makeup of net credit exposure with commodity exchange brokers	79
Makeup of net credit exposure with investment-grade counterparties	20
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	$ 25