Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
INCOME TAX EXPENSE	$ 65		$ 106	$ 122	$ 240
Effective tax rate	27.00%		33.00%	25.00%	33.00%
Estimated federal and state uncertain tax positions	3	72		3
Decrease in estimated prior year liabilities	238
Liabilities for uncertain tax positions	16			16
Interest charges related to uncertain tax positions		126
Interest charges related to uncertain tax position, LILO transaction		131
Liabilities for uncertain tax positions	13			13		44
Unrecognized tax benefits amount	6			6
Federal and State [Member]
Operating Loss Carryforwards [Line Items]
Estimated federal and state uncertain tax positions		238
CECONY [Member]
Operating Loss Carryforwards [Line Items]
INCOME TAX EXPENSE	57		75	209	209
Effective tax rate	27.00%		32.00%	33.00%	32.00%
Estimated federal and state uncertain tax positions		66
Liabilities for uncertain tax positions	7			7
Reduced interest expenses		5
Liabilities for uncertain tax positions	$ 7			$ 7		$ 36