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, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (16)	$ (13)	$ (13)
Included in earnings	(5)	(4)
Included in regulatory assets and liabilities	1	(5)
Settlements	7	3
Ending balance	(13)	(19)	(16)
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(2)	(2)
Included in earnings	(2)	0
Included in regulatory assets and liabilities	0	(3)
Settlements	2	0
Ending balance	$ (6)	$ (5)	$ (6)