Other Postretirement Benefits - Net Periodic Postretirement Benefit Costs (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 6	$ 14	$ 18
Interest cost on accumulated other postretirement benefit obligation	15	13	45	40
Expected return on plan assets	(19)	(19)	(58)	(58)
Recognition of net actuarial loss	14	16	43	48
Recognition of prior service cost	(5)	(7)	(14)	(20)
NET PERIODIC POSTRETIREMENT BENEFIT COST	10	9	30	28
Cost capitalized	(4)	(3)	(11)	(10)
Reconciliation to rate level	3	14	8	43
Cost charged to operating expenses	9	20	27	61
CECONY [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	11	14
Interest cost on accumulated other postretirement benefit obligation	13	12	39	34
Expected return on plan assets	(17)	(17)	(51)	(51)
Recognition of net actuarial loss	13	14	38	43
Recognition of prior service cost	(4)	(6)	(11)	(17)
NET PERIODIC POSTRETIREMENT BENEFIT COST	9	8	26	23
Cost capitalized	(4)	(3)	(10)	(9)
Reconciliation to rate level	1	12	2	37
Cost charged to operating expenses	$ 6	$ 17	$ 18	$ 51