Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ (198)	$ (57)
Effective Tax Rate	35.00%	23.00%
Corporate franchise tax rate	7.10%
Decrease in accumulated deferred tax liabilities	77
Decrease in regulatory asset	11
Increase in regulatory liability	66
Increase in estimated prior year liabilities	22
Uncertain tax positions	31
Interest charges related to uncertain tax positions		126
Interest charges related to uncertain tax position, LILO transaction		131
Reduced interest expense	13	5
Amount of interest and penalties in consolidated income statements	0		0
Uncertain tax positions expected to resolve in next twelve months	13
Unrecognized tax benefits, if recognized, would affect effective tax rate	31
Beginning January 1, 2016 [Member]
Operating Loss Carryforwards [Line Items]
Corporate franchise tax rate	6.50%
Federal and State [Member]
Operating Loss Carryforwards [Line Items]
Accrued liability for uncertain tax positions		249
Federal Taxes [Member]
Operating Loss Carryforwards [Line Items]
Uncertain tax positions expected to resolve in next twelve months, net of federal taxes	8
Unrecognized tax benefits, if recognized, would affect effective tax rate, net of federal taxes	20
Con Edison [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits		15
Decrease in accrued liability for uncertain tax positions		72
CECONY [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits	(184)	(152)
Decrease in accumulated deferred tax liabilities	72
Decrease in regulatory asset	10
Increase in regulatory liability	62
Decrease in accrued liability for uncertain tax positions		66
Uncertain tax positions	$ 0