Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 5	$ 10	$ 10
Interest cost on accumulated other postretirement benefit obligation	13	15	25	30
Expected return on plan assets	(20)	(19)	(39)	(38)
Recognition of net actuarial loss	8	14	16	28
Recognition of prior service cost	(5)	(5)	(10)	(10)
NET PERIODIC BENEFIT COST	1	10	2	20
Cost capitalized	(1)	(4)	(1)	(8)
Reconciliation to rate level	4	3	8	6
Cost charged to operating expenses	4	9	9	18
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4	7	7
Interest cost on accumulated other postretirement benefit obligation	11	13	22	26
Expected return on plan assets	(17)	(17)	(34)	(34)
Recognition of net actuarial loss	7	13	14	26
Recognition of prior service cost	(4)	(4)	(7)	(7)
NET PERIODIC BENEFIT COST	1	9	2	18
Cost capitalized	(1)	(4)	(1)	(7)
Reconciliation to rate level	2	1	3	1
Cost charged to operating expenses	$ 2	$ 6	$ 4	$ 12