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Commitments and Contingencies - Schedule of Accrued Liabilities for Environmental Exposures (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Commitments and Contingencies [Line Items]  
Beginning balance $ 30
Payments 2
Ending balance 28
Less amounts in Other Current Liabilities 4
Amounts in Other Deferred Credits 24
Potomac Electric Power Co [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 21
Payments 2
Ending balance 19
Less amounts in Other Current Liabilities 2
Amounts in Other Deferred Credits 17
Delmarva Power & Light Co/De [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 3
Ending balance 3
Less amounts in Other Current Liabilities 2
Amounts in Other Deferred Credits 1
Transmission and Distribution [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 19
Payments 2
Ending balance 17
Less amounts in Other Current Liabilities 3
Amounts in Other Deferred Credits 14
Transmission and Distribution [Member] | Potomac Electric Power Co [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 18
Payments 2
Ending balance 16
Less amounts in Other Current Liabilities 2
Amounts in Other Deferred Credits 14
Transmission and Distribution [Member] | Delmarva Power & Light Co/De [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 1
Ending balance 1
Less amounts in Other Current Liabilities 1
Legacy Generation Regulated [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 6
Ending balance 6
Less amounts in Other Current Liabilities 1
Amounts in Other Deferred Credits 5
Legacy Generation Regulated [Member] | Potomac Electric Power Co [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 3
Ending balance 3
Amounts in Other Deferred Credits 3
Legacy Generation Regulated [Member] | Delmarva Power & Light Co/De [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 2
Ending balance 2
Less amounts in Other Current Liabilities 1
Amounts in Other Deferred Credits 1
Legacy Generation Regulated [Member] | Atlantic City Electric Co [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 1
Ending balance 1
Amounts in Other Deferred Credits 1
Legacy Generation Non-Regulated [Member]
 
Commitments and Contingencies [Line Items]  
Beginning balance 5
Ending balance 5
Amounts in Other Deferred Credits $ 5