Income Tax - Schedule of Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Property basis differences	$ 6,555	$ 9,446
Regulatory assets:
Unrecognized pension and other postretirement costs	697	1,162
Future income tax	0	986
Environmental remediation costs	219	333
Deferred storm costs	11	23
Other regulatory assets	269	371
Equity investments	263	363
Total deferred tax liabilities	8,014	12,684
Deferred tax assets:
Accrued pension and other postretirement costs	264	581
Future income tax	698	0
Other regulatory liabilities	593	822
Superfund and other environmental costs	203	304
Asset retirement obligations	86	99
Loss carryforwards	95	59
Tax credits carryforward	658	498
Valuation allowance	(33)	(16)
Other	112	303
Total deferred tax assets	2,676	2,650
Net deferred tax liabilities	5,338	10,034
Unamortized investment tax credits	157	171
Net deferred tax liabilities and unamortized investment tax credits	5,495	10,205
CECONY
Deferred tax liabilities:
Property basis differences	5,968	8,620
Regulatory assets:
Unrecognized pension and other postretirement costs	656	1,104
Future income tax	0	940
Environmental remediation costs	187	287
Deferred storm costs	0	1
Other regulatory assets	241	321
Equity investments	0	0
Total deferred tax liabilities	7,052	11,273
Deferred tax assets:
Accrued pension and other postretirement costs	187	467
Future income tax	660	0
Other regulatory liabilities	524	728
Superfund and other environmental costs	176	265
Asset retirement obligations	79	92
Loss carryforwards	0	0
Tax credits carryforward	0	0
Valuation allowance	0	0
Other	148	312
Total deferred tax assets	1,774	1,864
Net deferred tax liabilities	5,278	9,409
Unamortized investment tax credits	28	41
Net deferred tax liabilities and unamortized investment tax credits	$ 5,306	$ 9,450