Income Tax - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Estimated federal and state uncertain tax positions	$ 72
Favorable impact on effective tax rate	15
Liabilities for uncertain tax positions	251	44
Interest charges related to uncertain tax positions	126
Interest charges related to uncertain tax position, LILO transaction	131
Acrued interest on uncertain tax positions	137	11
unrecognized tax benefits amount	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]
Estimated federal and state uncertain tax positions	66
Liabilities for uncertain tax positions	7	36
Reduced interest expenses	$ 5