Income Tax - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Taxes Disclosure [Line Items]
Income tax expense	$ 101	$ 102	$ 300	$ 300
Effective tax rate	32.00%	32.00%	34.00%	34.00%
Estimated liability for uncertain tax positions	$ 34		$ 34
Uncertain tax positions, reasonably possible to resolve within twelve months	25		25
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	16		16
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	34		34
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	22		22
CECONY
Income Taxes Disclosure [Line Items]
Income tax expense	$ 101	$ 78	$ 293	$ 262
Effective tax rate	32.00%	31.00%	34.00%	34.00%
Estimated liability for uncertain tax positions	$ 2		$ 2
Uncertain tax positions, reasonably possible to resolve within twelve months	2		2
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	$ 1		$ 1