Income Tax - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Taxes Disclosure [Line Items]
Income tax expense	$ 78.0	$ 55.0
Unprotected excess deferred federal income taxes amortization period	5 years
Estimated liability for uncertain tax positions	$ 14.0
Effective income tax rate reconciliation, uncertainty of taxes	3.0
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	14.0
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	13.0
CECONY
Income Taxes Disclosure [Line Items]
Income tax expense	114.0	$ 95.0
Estimated liability for uncertain tax positions	3.0
Effective income tax rate reconciliation, uncertainty of taxes	1.0
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	$ 3.0
Tax Year 2018 to 2013
Income Taxes Disclosure [Line Items]
Operating loss carryback			$ 29.0
Net tax refund			2.5
Tax Year 2013
Income Taxes Disclosure [Line Items]
Discrete income tax benefit			$ 4.0