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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 11	$ 24	$ 20	$ 9
Included in earnings	(3)	(2)	(15)	49
Included in regulatory assets and liabilities	0	3	1	7
Purchases	5	3	8	11
Settlements	0	(1)	(1)	(49)
Ending balance	13	27	13	27
CECONY
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	12	13	13	6
Included in earnings	(2)	(2)	(5)	9
Included in regulatory assets and liabilities	0	3	1	7
Purchases	2	2	4	9
Settlements	(1)	(2)	(2)	(17)
Ending balance	$ 11	$ 14	$ 11	$ 14