Income Tax - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Taxes Disclosure [Line Items]
Income taxes on operating income	$ 124	$ 101	$ 288	$ 300
Effective tax rate	35.00%	32.00%	35.00%	34.00%
Income taxes refund	$ 10		$ 10
Estimated liability for uncertain tax positions	36		36
Uncertain tax positions, reasonably possible to resolve within twelve months	27		27
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	18		18
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	36		36
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	24		24
CECONY
Income Taxes Disclosure [Line Items]
Income taxes on operating income	$ 84	$ 101	$ 264	$ 293
Effective tax rate	34.00%	32.00%	36.00%	34.00%
Estimated liability for uncertain tax positions	$ 4		$ 4
Uncertain tax positions, reasonably possible to resolve within twelve months	4		4
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	$ 3		$ 3