Other Postretirement Benefits - Periodic Postretirement Benefit Costs (Details) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Service cost - including administrative expenses	$ 2	$ 5	$ 15	$ 16
Interest cost on projected other postretirement benefit obligation	7	9	23	28
Expected return on plan assets	(17)	(16)	(51)	(49)
Recognition of net actuarial loss/(gain)	4	3	17	34
Recognition of prior service credit	0	(1)	(2)	(3)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	(4)	0	2	26
Cost capitalized	(3)	(2)	(9)	(7)
Reconciliation to rate level	7	2	7	(19)
Total credit recognized	0	0	0	0
CECONY | Subsidiaries
Defined Benefit Plan Disclosure [Line Items]
Service cost - including administrative expenses	1	4	11	12
Interest cost on projected other postretirement benefit obligation	6	8	19	23
Expected return on plan assets	(14)	(14)	(41)	(41)
Recognition of net actuarial loss/(gain)	3	3	13	32
Recognition of prior service credit	0	0	(1)	(1)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	(4)	1	1	25
Cost capitalized	(3)	(2)	(7)	(5)
Reconciliation to rate level	6	0	2	(24)
Total credit recognized	$ (1)	$ (1)	$ (4)	$ (4)