Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (4)	$ 11	$ 6	$ 20
Included in earnings	5	(3)	(2)	(15)
Included in regulatory assets and liabilities	1	0	(2)	1
Purchases	1	5	1	8
Settlements	2	0	2	(1)
Ending balance	5	13	5	13
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2	12	8	13
Included in earnings	0	(2)	(1)	(5)
Included in regulatory assets and liabilities	(1)	0	(5)	1
Purchases	1	2	1	4
Settlements	0	(1)	(1)	(2)
Ending balance	$ 2	$ 11	$ 2	$ 11