Income Tax - Schedule of Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities:
Property basis differences	$ 7,402	$ 6,555
Regulatory assets:
Unrecognized pension and other postretirement costs	627	697
Environmental remediation costs	227	219
Deferred storm costs	21	11
Other regulatory assets	273	269
Equity investments	102	263
Total deferred tax liabilities	8,652	8,014
Deferred tax assets:
Accrued pension and other postretirement costs	248	264
Future income tax	702	698
Other regulatory liabilities	632	593
Superfund and other environmental costs	218	203
Asset retirement obligations	114	86
Loss carryforwards	229	95
Tax credits carryforward	817	658
Valuation allowance	(33)	(33)
Other	53	112
Total deferred tax assets	2,980	2,676
Net deferred tax liabilities	5,672	5,338
Unamortized investment tax credits	148	157
Net deferred tax liabilities and unamortized investment tax credits	5,820	5,495
CECONY
Deferred tax liabilities:
Property basis differences	6,446	5,968
Regulatory assets:
Unrecognized pension and other postretirement costs	591	656
Environmental remediation costs	200	187
Deferred storm costs	0	0
Other regulatory assets	252	241
Equity investments	0	0
Total deferred tax liabilities	7,489	7,052
Deferred tax assets:
Accrued pension and other postretirement costs	180	187
Future income tax	662	660
Other regulatory liabilities	554	524
Superfund and other environmental costs	194	176
Asset retirement obligations	82	79
Loss carryforwards	0	0
Tax credits carryforward	0	0
Valuation allowance	0	0
Other	102	148
Total deferred tax assets	1,774	1,774
Net deferred tax liabilities	5,715	5,278
Unamortized investment tax credits	24	28
Net deferred tax liabilities and unamortized investment tax credits	$ 5,739	$ 5,306