Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Taxes Disclosure [Line Items]
INCOME TAX EXPENSE	$ 199	$ 198
Effective tax rate	35.00%	35.00%
Estimated liability for uncertain tax positions	34
Uncertain tax positions, reasonably possible to resolve within twelve months	25
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	16
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	34
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	22
CECONY [Member]
Income Taxes Disclosure [Line Items]
INCOME TAX EXPENSE	192	184
Effective tax rate	36.00%	36.00%
Estimated liability for uncertain tax positions	2
Uncertain tax positions, reasonably possible to resolve within twelve months	2
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	$ 1