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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 27	$ (6)	$ 9	$ (5)
Included in Earnings	(8)	(3)	41	5
Included in Regulatory Assets and Liabilities	(4)	(2)	4	1
Purchases	3	5	15	12
Issuances
Sales
Settlements	3	(8)	(48)	(27)
Transfer In/Out of Level 3
Ending Balance	21	(14)	21	(14)
CECONY [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	14	8	6	10
Included in Earnings	(3)	2	6	9
Included in Regulatory Assets and Liabilities	(4)	(1)	4
Purchases	3	5	13	10
Issuances
Sales
Settlements	1	(6)	(18)	(21)
Transfer In/Out of Level 3
Ending Balance	$ 11	$ 8	$ 11	$ 8