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Discontinued Operations - Income (Loss) from Discontinued Operations, Net of Income Taxes (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 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 $ 0 $ (11) $ 0 $ (330)
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   (15)   (334)
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   $ 4   $ 4