Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4	$ 5	$ 13	$ 15
Interest cost on accumulated other postretirement benefit obligation	12	13	36	38
Expected return on plan assets	(19)	(20)	(58)	(59)
Recognition of net actuarial loss	1	8	4	24
Recognition of prior service cost	(5)	(5)	(15)	(15)
NET PERIODIC BENEFIT COST	(7)	1	(20)	3
Cost capitalized	2	(1)	5	(2)
Reconciliation to rate level	7	4	20	12
Cost charged to operating expenses	2	4	5	13
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	4	10	11
Interest cost on accumulated other postretirement benefit obligation	10	11	30	32
Expected return on plan assets	(17)	(17)	(50)	(51)
Recognition of net actuarial loss	1	7	2	21
Recognition of prior service cost	(3)	(4)	(11)	(10)
NET PERIODIC BENEFIT COST	(6)	1	(19)	3
Cost capitalized	2	(1)	5	(2)
Reconciliation to rate level	6	2	19	5
Cost charged to operating expenses	$ 2	$ 2	$ 5	$ 6