Summary of Significant Accounting Policies - Research and Development Costs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Research and Development Expenses [Line Items]
Research and Development Costs	$ 22us-gaap_ResearchAndDevelopmentExpense	$ 18us-gaap_ResearchAndDevelopmentExpense	$ 21us-gaap_ResearchAndDevelopmentExpense
CECONY [Member]
Research and Development Expenses [Line Items]
Research and Development Costs	$ 20	$ 16	$ 19

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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