Asset Retirement Obligations - Accrued Liability for Asset Retirement Obligations and Regulatory Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Accrued liability - asset retirement obligations	$ 162	$ 159
Regulatory liabilities - allowance for cost of removal less salvage	1,393	1,202
Allowance for cost removal less salvage [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities - allowance for cost of removal less salvage	518	503
CECONY [Member]
Regulatory Liabilities [Line Items]
Accrued liability - asset retirement obligations	162	158
Regulatory liabilities - allowance for cost of removal less salvage	1,269	1,077
CECONY [Member] | Allowance for cost removal less salvage [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities - allowance for cost of removal less salvage	$ 433	$ 420