Income Tax - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
Income tax expense		$ 175,000,000	$ 270,000,000	$ 330,000,000	$ 599,000,000
TCJA, re-measurement of deferred tax assets						$ 42,000,000
Decrease in net deferred tax liabilities						16,000,000
Accrued regulatory liability for future income tax						58,000,000
Estimated liability for uncertain tax positions		11,000,000		11,000,000
Effective income tax rate reconciliation, uncertainty of taxes				10,000,000
Effective income tax rate reconciliation, uncertainty net of federal taxes				9,000,000
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate		11,000,000		11,000,000
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes		10,000,000		10,000,000
Interest expense for uncertain tax positions		0	0	0	0
Penalties for uncertain tax positions		0	0	0	0
Accrued interest on uncertain tax positions		0		0		0
Federal
Income Taxes Disclosure [Line Items]
NOL carryover						563,000,000
Settlement of uncertain federal tax positions	$ 3,000,000
Federal | Utilities
Income Taxes Disclosure [Line Items]
Additional depreciation						477,000,000
CECONY
Income Taxes Disclosure [Line Items]
Income tax expense		125,000,000	$ 242,000,000	274,000,000	$ 551,000,000
Decrease in net deferred tax liabilities						51,000,000
Accrued regulatory liability for future income tax						51,000,000
Estimated liability for uncertain tax positions		$ 5,000,000		5,000,000
Effective income tax rate reconciliation, uncertainty of taxes				$ 3,000,000
CECONY | Federal
Income Taxes Disclosure [Line Items]
NOL carryover						$ 153,000,000