Income Tax - Schedule of Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax liabilities:
Property basis differences	$ 7,985	$ 7,699
Regulatory assets:
Unrecognized pension and other postretirement costs	910	712
Environmental remediation costs	243	205
Deferred storm costs	31	22
Other regulatory assets	536	376
Operating lease right-of-use asset	220	231
Equity investments	46	104
Total deferred tax liabilities	9,971	9,349
Deferred tax assets:
Accrued pension and other postretirement costs	504	291
Future income tax	617	678
Other regulatory liabilities	656	702
Superfund and other environmental costs	241	206
Asset retirement obligations	178	135
Operating lease liabilities	211	231
Loss carryforwards	164	108
Tax credits carryforward	1,022	896
Valuation allowance	(22)	(31)
Other	59	47
Total deferred tax assets	3,630	3,263
Net deferred tax liabilities	6,341	6,086
Unamortized investment tax credits	134	141
Net deferred tax liabilities and unamortized investment tax credits	6,475	6,227
CECONY
Deferred tax liabilities:
Property basis differences	6,901	6,640
Regulatory assets:
Unrecognized pension and other postretirement costs	861	674
Environmental remediation costs	222	181
Deferred storm costs	0	0
Other regulatory assets	508	355
Operating lease right-of-use asset	165	169
Equity investments	0	0
Total deferred tax liabilities	8,657	8,019
Deferred tax assets:
Accrued pension and other postretirement costs	427	222
Future income tax	579	638
Other regulatory liabilities	570	622
Superfund and other environmental costs	219	183
Asset retirement obligations	143	102
Operating lease liabilities	165	170
Loss carryforwards	34	0
Tax credits carryforward	0	0
Valuation allowance	0	0
Other	127	103
Total deferred tax assets	2,264	2,040
Net deferred tax liabilities	6,393	5,979
Unamortized investment tax credits	18	21
Net deferred tax liabilities and unamortized investment tax credits	$ 6,411	$ 6,000