Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Details) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 5
Interest cost on accumulated other postretirement benefit obligation	11	11
Expected return on plan assets	(18)	(17)
Recognition of net actuarial loss/(gain)	2	1
Recognition of prior service cost	(2)	(4)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	(2)	(4)
Cost capitalized	(2)	2
Reconciliation to rate level	2	(1)
Cost charged to operating expenses	(2)	(3)
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3
Interest cost on accumulated other postretirement benefit obligation	9	10
Expected return on plan assets	(16)	(15)
Recognition of net actuarial loss/(gain)	1	(1)
Recognition of prior service cost	(1)	(3)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	(4)	(6)
Cost capitalized	(2)	2
Reconciliation to rate level	3	0
Cost charged to operating expenses	$ (3)	$ (4)