Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated OCI, net of taxes	$ (25)	$ (53)
OCI before reclassifications, net of tax	(26)	21
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	6	7
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	(20)	28	5
Accumulated OCI, net of taxes	(45)	(25)	(53)
CECONY [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated OCI, net of taxes	(6)	(9)
OCI before reclassifications, net of tax	(6)	2
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	1
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	(5)	3	(1)
Accumulated OCI, net of taxes	$ (11)	$ (6)	$ (9)

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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