Other Postretirement Benefits - Net Periodic Postretirement Benefit Costs (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 19	$ 23
Interest cost on accumulated other postretirement benefit obligation	51	60	54
Expected return on plan assets	(78)	(77)	(77)
Recognition of net actuarial loss	31	57	65
Recognition of prior service cost	(20)	(19)	(27)
NET PERIODIC BENEFIT COST	4	40	38
Cost capitalized	(2)	(15)	(15)
Reconciliation to rate level	14	10	58
Cost charged to operating expenses	16	35	81
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	15	15	18
Interest cost on accumulated other postretirement benefit obligation	43	52	46
Expected return on plan assets	(68)	(68)	(68)
Recognition of net actuarial loss	28	51	57
Recognition of prior service cost	(14)	(15)	(23)
NET PERIODIC BENEFIT COST	4	35	30
Cost capitalized	(2)	(14)	(12)
Reconciliation to rate level	6	2	50
Cost charged to operating expenses	$ 8	$ 23	$ 68