Other Postretirement Benefits - Schedule of Funded Status (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CHANGE IN BENEFIT OBLIGATION
Projected benefit obligation at beginning of year	$ 1,411	$ 1,395	$ 1,454
Service cost	20	19	23
Interest cost on accumulated other postretirement benefit obligation	51	60	54
Amendments	0	(12)	0
Net actuarial loss/(gain)	(103)	47	(42)
Benefits paid and administrative expenses	(127)	(134)	(136)
Participant contributions	35	36	38
Medicare prescription subsidy	0	0	4
PROJECTED BENEFIT OBLIGATION AT END OF YEAR	1,287	1,411	1,395
CHANGE IN PLAN ASSETS
Fair value of plan assets at beginning of year	1,084	1,113	1,047
Actual return on plan assets	(6)	59	153
Employer contributions	6	7	9
EGWP payments	28	12	8
Participant contributions	35	36	38
Benefits paid	(153)	(143)	(142)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	994	1,084	1,113
FUNDED STATUS	(293)	(327)	(282)
Unrecognized net loss	28	78	70
Unrecognized prior service costs	(51)	(71)	(78)
CECONY
CHANGE IN BENEFIT OBLIGATION
Projected benefit obligation at beginning of year	1,203	1,198	1,238
Service cost	15	15	18
Interest cost on accumulated other postretirement benefit obligation	43	52	46
Amendments	0	0	0
Net actuarial loss/(gain)	(85)	28	(20)
Benefits paid and administrative expenses	(117)	(125)	(126)
Participant contributions	34	35	38
Medicare prescription subsidy	0	0	4
PROJECTED BENEFIT OBLIGATION AT END OF YEAR	1,093	1,203	1,198
CHANGE IN PLAN ASSETS
Fair value of plan assets at beginning of year	950	977	922
Actual return on plan assets	(4)	54	134
Employer contributions	6	7	9
EGWP payments	26	11	7
Participant contributions	34	35	38
Benefits paid	(142)	(134)	(133)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	870	950	977
FUNDED STATUS	(223)	(253)	(221)
Unrecognized net loss	4	45	54
Unrecognized prior service costs	$ (32)	$ (46)	$ (61)