Other Postretirement Benefits - Net Periodic Postretirement Benefit Costs (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 6
Interest cost on accumulated other postretirement benefit obligation	15	14
Expected return on plan assets	(19)	(19)
Recognition of net actuarial loss	14	16
Recognition of prior service cost	(5)	(7)
NET PERIODIC POSTRETIREMENT BENEFIT COST	10	10
Cost capitalized	(4)	(3)
Reconciliation to rate level	3	12
Cost charged to operating expenses	9	19
CECONY [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5
Interest cost on accumulated other postretirement benefit obligation	13	12
Expected return on plan assets	(17)	(17)
Recognition of net actuarial loss	13	14
Recognition of prior service cost	(4)	(6)
NET PERIODIC POSTRETIREMENT BENEFIT COST	9	8
Cost capitalized	(3)	(2)
Reconciliation to rate level		11
Cost charged to operating expenses	$ 6	$ 17