Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4	$ 5
Interest cost on accumulated other postretirement benefit obligation	12	13
Expected return on plan assets	(19)	(20)
Recognition of net actuarial loss	1	8
Recognition of prior service cost	(5)	(5)
NET PERIODIC BENEFIT COST	(7)	1
Cost capitalized	2	(1)
Reconciliation to rate level	7	4
Cost charged to operating expenses	2	4
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	4
Interest cost on accumulated other postretirement benefit obligation	10	11
Expected return on plan assets	(17)	(17)
Recognition of net actuarial loss	1	7
Recognition of prior service cost	(4)	(4)
NET PERIODIC BENEFIT COST	(7)	1
Cost capitalized	2	(1)
Reconciliation to rate level	7	2
Cost charged to operating expenses	$ 2	$ 2