Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended				9 Months Ended
	Mar. 31, 2014	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating Loss Carryforwards [Line Items]
Income tax benefits		$ 259		$ 250		$ 559	$ 373
Effective tax rate		37.00%		35.00%		36.00%	31.00%
Increase (decrease) in income tax expense					(15)
Corporate franchise tax rate	7.10%
Decrease in accumulated deferred tax liabilities						72
Decrease in regulatory asset						11
Increase in regulatory liability						61
Federal income tax benefits				13
Increase in estimated prior year liabilities			25
Uncertain tax positions		34				34
Uncertain tax positions expected to resolve in next twelve months		25				25
Unrecognized tax benefits, if recognized, would reduce effective tax rate		34				34
Federal Taxes [Member]
Operating Loss Carryforwards [Line Items]
Unrecognized tax benefits, if recognized, would reduce effective tax rate, net of federal taxes		22				22
Uncertain tax positions expected to resolve in next twelve months, net of federal taxes		16				16
Beginning January 1, 2016 [Member]
Operating Loss Carryforwards [Line Items]
Corporate franchise tax rate	6.50%
CECONY [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits		233		222		496	431
Effective tax rate		37.00%		36.00%		35.00%	34.00%
Decrease in accumulated deferred tax liabilities						67
Decrease in regulatory asset						10
Increase in regulatory liability						57
Federal income tax benefits				7
Uncertain tax positions		2				2
Uncertain tax positions expected to resolve in next twelve months		2				2
CECONY [Member] | Federal Taxes [Member]
Operating Loss Carryforwards [Line Items]
Uncertain tax positions expected to resolve in next twelve months, net of federal taxes		1				1
Con Edison [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits				$ 6			$ 6