Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014 Contract
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 151	$ 151
Makeup of net credit exposure independent system operators		54
Makeup of net credit exposure with commodity exchange brokers	63	63
Makeup of net credit exposure with investment-grade counterparties	31	31
Makeup of net credit exposure non-investment grade/non-rated counterparties		3
Number of contracts		1,335
Expiration date of interest rate swap		Oct. 01, 2014
Increase in the fair value of interest rate swap	1	2
Interest Rate Swap [Member]
Investment Holdings [Line Items]
Derivative, fixed interest rate	6.09%	6.09%
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	28	28
Makeup of net credit exposure with commodity exchange brokers	20	20
Makeup of net credit exposure with investment-grade counterparties	$ 8	$ 8
Number of contracts		565