Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4	$ 5	$ 9	$ 10
Interest cost on accumulated other postretirement benefit obligation	12	13	24	25
Expected return on plan assets	(19)	(20)	(38)	(39)
Recognition of net actuarial loss	1	8	2	16
Recognition of prior service cost	(5)	(5)	(10)	(10)
NET PERIODIC BENEFIT COST	(7)	1	(13)	2
Cost capitalized	2	(1)	3	(1)
Reconciliation to rate level	7	4	14	8
Cost charged to operating expenses	2	4	4	9
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	4	7	7
Interest cost on accumulated other postretirement benefit obligation	10	11	20	22
Expected return on plan assets	(17)	(17)	(34)	(34)
Recognition of net actuarial loss	1	7	1	14
Recognition of prior service cost	(3)	(4)	(7)	(7)
NET PERIODIC BENEFIT COST	(6)	1	(13)	2
Cost capitalized	2	(1)	3	(1)
Reconciliation to rate level	6	2	14	3
Cost charged to operating expenses	$ 2	$ 2	$ 4	$ 4