Income Tax - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Taxes Disclosure [Line Items]
Income taxes on operating income		$ 314	$ 249	$ 602	$ 548
Effective tax rate		39.00%	37.00%	37.00%	35.00%
Estimated liability for uncertain tax positions		$ 37		$ 37
Uncertain tax positions, reasonably possible to resolve within twelve months		29		29
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes		20		20
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate		37		37
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes		25		25
CECONY
Income Taxes Disclosure [Line Items]
Income taxes on operating income		$ 226	$ 222	$ 491	$ 515
Effective tax rate		37.00%	37.00%	36.00%	35.00%
Estimated liability for uncertain tax positions		$ 5		$ 5
Uncertain tax positions, reasonably possible to resolve within twelve months		5		5
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes		4		4
Subsequent Event
Income Taxes Disclosure [Line Items]
Income tax refund received	$ 19
Tax Year 2015 | Subsequent Event
Income Taxes Disclosure [Line Items]
Income taxes refund	$ 16
Tax Year 2016
Income Taxes Disclosure [Line Items]
Income taxes refund		$ 32		$ 32