Other Postretirement Benefits - Schedule of Funded Status (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CHANGE IN BENEFIT OBLIGATION
Benefit obligation at beginning of year	$ 1,756	$ 1,642	$ 1,697
Service cost	26	25	24
Interest cost on accumulated postretirement benefit obligation	73	83	91
Amendments	(127)
Net actuarial loss/(gain)	(175)	109	(68)
Benefits paid and administrative expenses	(146)	(144)	(138)
Participant contributions	37	33	29
Medicare prescription subsidy	10	8	7
BENEFIT OBLIGATION AT END OF YEAR	1,454	1,756	1,642
CHANGE IN PLAN ASSETS
Beginning Balance as of January 1, 2011	947	942	866
Actual return on plan assets	124	20	89
Employer contributions	83	84	96
Participant contributions	37	33	29
Benefits paid	(144)	(132)	(138)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	1,047	947	942
FUNDED STATUS	(407)	(809)	(700)
Unrecognized net loss	251	563	483
Unrecognized prior service costs	(105)	(1)	(10)
Unrecognized net transition liability at January 1, 1993		4	7
CECONY [Member]
CHANGE IN BENEFIT OBLIGATION
Benefit obligation at beginning of year	1,511	1,426	1,495
Service cost	21	20	19
Interest cost on accumulated postretirement benefit obligation	63	72	80
Amendments	(89)
Net actuarial loss/(gain)	(178)	86	(77)
Benefits paid and administrative expenses	(134)	(132)	(126)
Participant contributions	36	32	28
Medicare prescription subsidy	8	7	7
BENEFIT OBLIGATION AT END OF YEAR	1,238	1,511	1,426
CHANGE IN PLAN ASSETS
Beginning Balance as of January 1, 2011	840	839	777
Actual return on plan assets	109	19	78
Employer contributions	71	74	85
Participant contributions	36	32	28
Benefits paid	(134)	(124)	(129)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	922	840	839
FUNDED STATUS	(316)	(671)	(587)
Unrecognized net loss	197	496	440
Unrecognized prior service costs	(84)	(15)	(26)
Unrecognized net transition liability at January 1, 1993		$ 4	$ 7