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
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1	$ 1
Included in earnings	2	0
Included in regulatory assets and liabilities	2	2
Settlements	(1)	0
Transfer out of level 3	(1)	0
Ending balance	3	3
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	4	1
Included in earnings	2	0
Included in regulatory assets and liabilities	(1)	0
Settlements	(2)	0
Transfer out of level 3	(1)	0
Ending balance	$ 2	$ 1