Discontinued Operations - Income (Loss) from Discontinued Operations, Net of Income Taxes (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 (Loss) from Discontinued Operations, net of Income Taxes	$ 9	$ 9	$ (322)
Cross-Border Energy Lease Investments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Discontinued Operations, net of Income Taxes		$ 9	(327)
Pepco Energy Services' Retail Electric and Natural Gas Supply Businesses [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Discontinued Operations, net of Income Taxes			$ 5