Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 11	$ 632
Accrued liability for uncertain tax position	72
Uncertain tax positions	86
Increase in current liabilities	44
Reduction to current deferred tax assets	42
Unrecognized tax benefits	86	130	93	86
CECONY [Member]
Operating Loss Carryforwards [Line Items]
Accrued liability for uncertain tax position	66
Uncertain tax positions	74
Increase in current liabilities	36
Reduction to current deferred tax assets	38
Unrecognized tax benefits	74	114	79	92
New York State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	284
Unrecognized tax benefits	7
New York State [Member] | CECONY [Member]
Operating Loss Carryforwards [Line Items]
Unrecognized tax benefits	$ 1