Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended
	Jun. 30, 2013	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 CECONY [Member]	Dec. 31, 2013 CECONY [Member]	Dec. 31, 2012 CECONY [Member]	Dec. 31, 2011 CECONY [Member]	Dec. 31, 2013 Con Edison [Member]	Dec. 31, 2013 New York State [Member]	Jan. 31, 2013 Federal and State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward			$ 529								$ 213
Net operating loss carryforward expire date											2019 through 2033
Deferred tax asset			8,344	8,076			7,732	7,259			12
Accrued liability for uncertain tax position			72	72			66	66				249
Decrease in estimated prior year liabilities	249		86	57	43		74	52	42
Income tax benefits		13	476	600	600	7	520	529	558	6
Uncertain tax positions			9
Increase in noncurrent liabilities			9
Interest charges related to uncertain tax positions			121
Reduced interest expenses			10
Interest charges related to uncertain tax position, LILO transaction			131
Amount of interest and penalties in their consolidated income statements				0	0
Amount of interest and penalties in their consolidated balance sheets			0	0
Unrecognized tax benefits amount			$ 9