Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance				$ 29
Included in Earnings				(80)
Included in Regulatory Assets and Liabilities				(10)
Purchases				11
Issuances
Sales
Settlements				82
Transfer In/Out of Level 3				(93)
Ending Balance		(61)		(61)
Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	14	(93)	(5)	(62)
Included in Earnings	(21)	(24)	8	(82)
Included in Regulatory Assets and Liabilities	(2)	4	3	(13)
Purchases	2	5	7	11
Issuances
Sales
Settlements	1	43	(19)	81
Transfer In/Out of Level 3		4		4
Ending Balance	(6)	(61)	(6)	(61)
Interest rate contract [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance				(8)
Included in Earnings				(1)
Issuances
Sales
Settlements				1
Transfer In/Out of Level 3				8
Other assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance				99
Included in Earnings				3
Included in Regulatory Assets and Liabilities				3
Issuances
Sales
Transfer In/Out of Level 3				(105)
CECONY [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance				83
Included in Earnings				(13)
Included in Regulatory Assets and Liabilities				(1)
Purchases				8
Issuances
Sales
Settlements				6
Transfer In/Out of Level 3				(93)
Ending Balance		(10)		(10)
CECONY [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	11	(13)	10	(7)
Included in Earnings	(2)	(11)	7	(16)
Included in Regulatory Assets and Liabilities		4	1	(3)
Purchases	1	2	5	8
Issuances
Sales
Settlements	(2)	6	(15)	6
Transfer In/Out of Level 3		2		2
Ending Balance	8	(10)	8	(10)
CECONY [Member] | Other assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance				90
Included in Earnings				3
Included in Regulatory Assets and Liabilities				2
Issuances
Sales
Transfer In/Out of Level 3				$ (95)