Fair Value Measurements - Reconciliation of Assets and Liabilities Measured at Level 3 Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (13)	$ (19)	$ (16)	$ (13)	$ (13)
Included in earnings	(2)	0	(7)	(4)
Included in regulatory assets and liabilities	0	(27)	1	(31)
Settlements	1	0	8	2
Ending balance	(14)	(46)	(14)	(46)	(16)
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(5)	(6)	(2)	(2)
Included in earnings	(1)	1	(3)	1
Included in regulatory assets and liabilities	1	(25)	(1)	(28)
Settlements	(1)	(1)	3	(1)
Ending balance	$ (7)	$ (30)	$ (7)	$ (30)	$ (6)