Pension Benefits - Total Periodic Benefit Costs (Detail) - Pension Benefits - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	$ 74	$ 57	$ 223	$ 170
Interest cost on projected benefit obligation	144	143	431	429
Expected return on plan assets	(222)	(208)	(664)	(624)
Recognition of net actuarial loss	194	154	581	464
Recognition of prior service costs	1	1	3	3
NET PERIODIC BENEFIT COST	191	147	574	442
Amortization of regulatory asset	1	1	2	2
TOTAL PERIODIC BENEFIT COST	192	148	576	444
Cost capitalized	(80)	(57)	(224)	(166)
Reconciliation to rate level	(14)	30	(56)	86
Cost charged to operating expenses	98	121	296	364
CECONY
Defined Benefit Plan Disclosure [Line Items]
Service cost – including administrative expenses	70	53	209	158
Interest cost on projected benefit obligation	135	134	404	402
Expected return on plan assets	(210)	(198)	(630)	(592)
Recognition of net actuarial loss	183	146	550	439
Recognition of prior service costs	0	1	1	2
NET PERIODIC BENEFIT COST	178	136	534	409
Amortization of regulatory asset	1	1	2	2
TOTAL PERIODIC BENEFIT COST	179	137	536	411
Cost capitalized	(76)	(54)	(214)	(156)
Reconciliation to rate level	(14)	28	(56)	78
Cost charged to operating expenses	$ 89	$ 111	$ 266	$ 333