Discontinued Operations - Operating Results for Retail Electric and Natural Gas Supply Businesses (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations, net of income taxes	$ 9	$ 9	$ (322)
Discontinued Operations [Member] \| Pepco Energy Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenue			84
Income from operations of discontinued operations, net of income taxes			4
Net gains associated with accelerated disposition of retail electric and natural gas contracts, net of income taxes			1
Income from discontinued operations, net of income taxes			$ 5

Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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