Pension Benefits - Total Periodic Benefit Costs (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	$ 263	$ 275	$ 297
Interest cost on projected benefit obligation	591	596	575
Expected return on plan assets	(968)	(947)	(886)
Recognition of net actuarial loss	595	596	775
Recognition of prior service costs	(17)	4	4
NET PERIODIC BENEFIT COST	464	524	765
Amortization of regulatory asset	0	0	1
TOTAL PERIODIC BENEFIT COST	464	524	766
Cost capitalized	(181)	(214)	(301)
Reconciliation to rate level	(34)	54	(74)
Cost charged to operating expenses	249	364	391
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	246	258	279
Interest cost on projected benefit obligation	554	559	538
Expected return on plan assets	(917)	(898)	(840)
Recognition of net actuarial loss	563	565	734
Recognition of prior service costs	(19)	2	2
NET PERIODIC BENEFIT COST	427	486	713
Amortization of regulatory asset	0	0	1
TOTAL PERIODIC BENEFIT COST	427	486	714
Cost capitalized	(169)	(203)	(285)
Reconciliation to rate level	(41)	58	(74)
Cost charged to operating expenses	$ 217	$ 341	355
Increase in CECONY's pension obligation			$ 45