Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended				6 Months Ended
	Mar. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating Loss Carryforwards [Line Items]
Increase (decrease) in income tax expense		$ 37			$ (15)	$ 178
Effective tax rate		32.00%		27.00%		34.00%	25.00%
Corporate franchise tax rate	7.10%
Decrease in accumulated deferred tax liabilities						80
Decrease in regulatory asset						11
Increase in regulatory liability						69
Increase in estimated prior year liabilities			22
Uncertain tax positions		31				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
Uncertain tax positions expected to resolve in next twelve months		13				13
Unrecognized tax benefits, if recognized, would reduce effective tax rate		31				31
Federal Taxes [Member]
Operating Loss Carryforwards [Line Items]
Uncertain tax positions expected to resolve in next twelve months, net of federal taxes		8				8
Unrecognized tax benefits, if recognized, would reduce effective tax rate, net of federal taxes		20				20
Beginning January 1, 2016 [Member]
Operating Loss Carryforwards [Line Items]
Corporate franchise tax rate	6.50%
CECONY [Member]
Operating Loss Carryforwards [Line Items]
Increase (decrease) in income tax expense		21				53
Effective tax rate		31.00%		27.00%		34.00%	33.00%
Decrease in accumulated deferred tax liabilities						75
Decrease in regulatory asset						10
Increase in regulatory liability						65
Uncertain tax positions		$ 0				$ 0