Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Detail) (Commodity [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 24	$ 14	$ 9	$ (5)
Included in Earnings	(2)	(21)	49	8
Included in Regulatory Assets and Liabilities	3	(2)	7	3
Purchases	3	2	11	7
Issuances
Sales
Settlements	(1)	1	(49)	(19)
Transfer In/Out of Level 3
Ending Balance	27	(6)	27	(6)
CECONY [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	13	11	6	10
Included in Earnings	(2)	(2)	9	7
Included in Regulatory Assets and Liabilities	3		7	1
Purchases	2	1	9	5
Issuances
Sales
Settlements	(2)	(2)	(17)	(15)
Transfer In/Out of Level 3
Ending Balance	$ 14	$ 8	$ 14	$ 8