Income Tax - Schedule of Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities:
Property basis differences	$ 7,699	$ 7,402
Regulatory assets:
Unrecognized pension and other postretirement costs	712	627
Environmental remediation costs	205	227
Deferred storm costs	22	21
Other regulatory assets	376	273
Operating lease right-of-use asset	231	0
Equity investments	104	102
Total deferred tax liabilities	9,349	8,652
Deferred tax assets:
Accrued pension and other postretirement costs	291	248
Future income tax	678	702
Other regulatory liabilities	702	632
Superfund and other environmental costs	206	218
Asset retirement obligations	135	114
Operating lease liabilities	231	0
Loss carryforwards	108	229
Tax credits carryforward	896	817
Valuation allowance	(40)	(33)
Other	56	53
Total deferred tax assets	3,263	2,980
Net deferred tax liabilities	6,086	5,672
Unamortized investment tax credits	141	148
Net deferred tax liabilities and unamortized investment tax credits	6,227	5,820
CECONY
Deferred tax liabilities:
Property basis differences	6,640	6,446
Regulatory assets:
Unrecognized pension and other postretirement costs	674	591
Environmental remediation costs	181	200
Deferred storm costs	0	0
Other regulatory assets	355	252
Operating lease right-of-use asset	169	0
Equity investments	0	0
Total deferred tax liabilities	8,019	7,489
Deferred tax assets:
Accrued pension and other postretirement costs	222	180
Future income tax	638	662
Other regulatory liabilities	622	554
Superfund and other environmental costs	183	194
Asset retirement obligations	102	82
Operating lease liabilities	170	0
Loss carryforwards	0	0
Tax credits carryforward	0	0
Valuation allowance	0	0
Other	103	102
Total deferred tax assets	2,040	1,774
Net deferred tax liabilities	5,979	5,715
Unamortized investment tax credits	21	24
Net deferred tax liabilities and unamortized investment tax credits	$ 6,000	$ 5,739