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Stock-Based Compensation Plans - Narrative (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common Stock Narrative Information [Line Items]      
Obligations of outstanding restricted stock not settled $ 108 $ 101  
Settlement of restricted stock 88 68 $ 75
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options 41    
Classification Of Obligations Related To Outstanding Performance Share Awards Not Yet Settled Deferred Credits And Other Liabilities [1] $ 100 $ 52  
Pepco Holdings LLC [Member]      
Common Stock Narrative Information [Line Items]      
Settlement of restricted stock     $ 6
Performance Shares [Member]      
Common Stock Narrative Information [Line Items]      
Granted, Weighted Average Grant Date Fair Value (in usd per share) $ 35.00 [2] $ 28.85 $ 35.88
Percentage to be settled as common stock 50.00%    
Percentage to be settled as cash 50.00%    
Percentage to be settled as cash if ownership requirements are met 100.00%    
Performance Shares [Member] | Pepco Holdings LLC [Member]      
Common Stock Narrative Information [Line Items]      
Granted, Weighted Average Grant Date Fair Value (in usd per share)     26.10
Employee Stock Option [Member]      
Common Stock Narrative Information [Line Items]      
Requisite service period 4 years    
Restricted Stock [Member]      
Common Stock Narrative Information [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 8 months 12 days    
Restricted Stock Units (RSUs) [Member]      
Common Stock Narrative Information [Line Items]      
Granted, Weighted Average Grant Date Fair Value (in usd per share) $ 34.98 $ 28.14 36.55
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options $ 51    
Restricted Stock Units (RSUs) [Member] | Pepco Holdings LLC [Member]      
Common Stock Narrative Information [Line Items]      
Granted, Weighted Average Grant Date Fair Value (in usd per share)     $ 27.40
Performance based RSU [Member] | Pepco Holdings LLC [Member]      
Common Stock Narrative Information [Line Items]      
Settlement of restricted stock     $ 15
Granted, Weighted Average Grant Date Fair Value (in usd per share)     $ 26.08
Minimum [Member] | Restricted Stock [Member]      
Common Stock Narrative Information [Line Items]      
Requisite service period 3 years    
Maximum [Member] | Restricted Stock [Member]      
Common Stock Narrative Information [Line Items]      
Requisite service period 5 years    
LTIP [Member]      
Common Stock Narrative Information [Line Items]      
Shares authorized 13    
[1] Represents the long-term liability related to performance share awards expected to be settled in cash.
[2] (a)As of December 31, 2017, $41 million of total unrecognized compensation costs related to nonvested performance shares are expected to be recognized over the remaining weighted-average period of 1.5 years.