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
	Mar. 31, 2014	Mar. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 9	$ (5)
Included in Earnings	50	31
Included in Regulatory Assets and Liabilities	4	5
Purchases	8	4
Issuances
Sales
Settlements	(47)	(21)
Transfer In/Out of Level 3
Ending Balance	24	14
CECONY [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	6	10
Included in Earnings	11	10
Included in Regulatory Assets and Liabilities	4	1
Purchases	7	4
Issuances
Sales
Settlements	(15)	(14)
Transfer In/Out of Level 3
Ending Balance	$ 13	$ 11