Other Postretirement Benefits - Net Periodic Postretirement Benefit Costs (Details) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 18	$ 20
Interest cost on accumulated other postretirement benefit obligation	46	48	51
Expected return on plan assets	(69)	(77)	(78)
Recognition of net actuarial loss	2	5	31
Recognition of prior service cost	(17)	(20)	(20)
NET PERIODIC BENEFIT COST	(18)	(26)	4
Cost capitalized	8	11	(2)
Reconciliation to rate level	(4)	22	14
Cost charged to operating expenses	(14)	7	16
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	13	15
Interest cost on accumulated other postretirement benefit obligation	38	40	43
Expected return on plan assets	(61)	(67)	(68)
Recognition of net actuarial loss	(3)	3	28
Recognition of prior service cost	(11)	(14)	(14)
NET PERIODIC BENEFIT COST	(24)	(25)	4
Cost capitalized	10	10	(2)
Reconciliation to rate level	(2)	22	6
Cost charged to operating expenses	$ (16)	$ 7	$ 8