Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 3	$ (4)	$ 1	$ 6
Included in earnings	2	5	1	(2)
Included in regulatory assets and liabilities	(11)	1	(9)	(2)
Purchases	0	1	1	1
Settlements	(4)	2	(4)	2
Ending balance	(10)	5	(10)	5
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	2	1	8
Included in earnings	1	0	0	(1)
Included in regulatory assets and liabilities	(7)	(1)	(7)	(5)
Purchases	0	1	1	1
Settlements	(1)	0	(1)	(1)
Ending balance	$ (6)	$ 2	$ (6)	$ 2