Pension Benefits - Schedule of Funded Status (Detail) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CHANGE IN PROJECTED BENEFIT OBLIGATION
Projected benefit obligation at beginning of year	$ 15,081	$ 12,197	$ 13,406
Service cost – excluding administrative expenses	293	221	259
Interest cost on projected benefit obligation	575	572	537
Net actuarial (gain)/loss	(996)	2,641	(1,469)
Plan amendments	0	6	0
Benefits paid	(576)	(556)	(536)
PROJECTED BENEFIT OBLIGATION AT END OF YEAR	14,377	15,081	12,197
CHANGE IN PLAN ASSETS
Fair value of plan assets at beginning of year	11,495	10,755	9,135
Actual return on plan assets	126	752	1,310
Employer contributions	750	578	879
Benefits paid	(576)	(556)	(536)
Administrative expenses	(36)	(34)	(33)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	11,759	11,495	10,755
FUNDED STATUS	(2,618)	(3,586)	(1,442)
Unrecognized net loss	3,909	4,888	2,759
Unrecognized prior service costs	16	20	17
Accumulated benefit obligation	12,909	13,454	11,004
CECONY
CHANGE IN PROJECTED BENEFIT OBLIGATION
Projected benefit obligation at beginning of year	14,137	11,429	12,572
Service cost – excluding administrative expenses	274	206	241
Interest cost on projected benefit obligation	538	536	503
Net actuarial (gain)/loss	(931)	2,484	(1,388)
Plan amendments	0	0	0
Benefits paid	(536)	(518)	(499)
PROJECTED BENEFIT OBLIGATION AT END OF YEAR	13,482	14,137	11,429
CHANGE IN PLAN ASSETS
Fair value of plan assets at beginning of year	10,897	10,197	8,668
Actual return on plan assets	118	715	1,241
Employer contributions	697	535	819
Benefits paid	(536)	(518)	(499)
Administrative expenses	(35)	(32)	(32)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	11,141	10,897	10,197
FUNDED STATUS	(2,341)	(3,240)	(1,232)
Unrecognized net loss	3,704	4,616	2,617
Unrecognized prior service costs	3	4	6
Accumulated benefit obligation	$ 12,055	$ 12,553	$ 10,268