Pension Benefits - Total Periodic Benefit Costs (Detail) - Pension Benefits - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	$ 69	$ 74	$ 207	$ 223
Interest cost on projected benefit obligation	149	144	447	431
Expected return on plan assets	(237)	(222)	(711)	(664)
Recognition of net actuarial loss	149	194	447	581
Recognition of prior service costs	1	1	3	3
NET PERIODIC BENEFIT COST	131	191	393	574
Amortization of regulatory asset	0	1	0	2
TOTAL PERIODIC BENEFIT COST	131	192	393	576
Cost capitalized	(51)	(80)	(157)	(224)
Reconciliation to rate level	10	(14)	35	(56)
Cost charged to operating expenses	90	98	271	296
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	65	70	194	209
Interest cost on projected benefit obligation	140	135	419	404
Expected return on plan assets	(225)	(210)	(674)	(630)
Recognition of net actuarial loss	141	183	424	550
Recognition of prior service costs	0	0	1	1
NET PERIODIC BENEFIT COST	121	178	364	534
Amortization of regulatory asset	0	1	0	2
TOTAL PERIODIC BENEFIT COST	121	179	364	536
Cost capitalized	(49)	(76)	(148)	(214)
Reconciliation to rate level	13	(14)	39	(56)
Cost charged to operating expenses	$ 85	$ 89	$ 255	$ 266