Fair Value Measurements - Reconciliation of Assets and Liabilities Measured at Level 3 Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (13)	$ 1	$ 1
Included in earnings	(4)	2
Included in regulatory assets and liabilities	(5)	2
Settlements	3	(1)
Transfer out of level 3	0	(1)
Ending balance	(19)	3	(13)
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	4	4
Included in earnings	0	2
Included in regulatory assets and liabilities	(3)	(1)
Settlements	0	(2)
Transfer out of level 3	0	(1)
Ending balance	$ (5)	$ 2	$ (2)