Income Tax - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Taxes Disclosure [Line Items]
Income tax expense	$ 164	$ 199
Effective tax rate	35.00%	35.00%
Income taxes refund	$ 10
Estimated liability for uncertain tax positions	36
Uncertain tax positions, reasonably possible to resolve within twelve months	27
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	18
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	36
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	24
CECONY
Income Taxes Disclosure [Line Items]
Income tax expense	$ 180	$ 192
Effective tax rate	37.00%	36.00%
Estimated liability for uncertain tax positions	$ 4
Uncertain tax positions, reasonably possible to resolve within twelve months	4
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	$ 3