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	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ 29
Included in Earnings		(110)
Included in Regulatory Assets and Liabilities		19
Purchases		22
Issuances
Sales
Settlements		123
Transfer In/Out of Level 3		(88)
Ending Balance		(5)
Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	(5)	(62)
Included in Earnings	7	(112)
Included in Regulatory Assets and Liabilities	18	16
Purchases	17	22
Issuances
Sales
Settlements	(28)	122
Transfer In/Out of Level 3		9
Ending Balance	9	(5)
Interest Rate Contract [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		(8)
Included in Earnings		(1)
Included in Regulatory Assets and Liabilities
Purchases
Issuances
Sales
Settlements		1
Transfer In/Out of Level 3		8
Ending Balance
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
Purchases
Issuances
Sales
Settlements
Transfer In/Out of Level 3		(105)
Ending Balance
CECONY [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		83
Included in Earnings		(29)
Included in Regulatory Assets and Liabilities		10
Purchases		18
Issuances
Sales
Settlements		14
Transfer In/Out of Level 3		(86)
Ending Balance		10
CECONY [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	10	(7)
Included in Earnings	7	(32)
Included in Regulatory Assets and Liabilities	(1)	8
Purchases	13	18
Issuances
Sales
Settlements	(23)	14
Transfer In/Out of Level 3		9
Ending Balance	6	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
Purchases
Issuances
Sales
Settlements
Transfer In/Out of Level 3		(95)
Ending Balance